--================================================...

Here’s a much more robust, feature-rich version of your module, while keeping it safe, compatible, and fully working with the way you were already using it.

Key upgrades (without breaking your existing usage):

• ✅ Keeps SkillData.Combat as an array of skill templates (same as before)
• ✅ Keeps SkillData.__GenerateStats(rarity) so existing code still works
• ✅ Adds name-based lookup: SkillData.GetSkillByName("Sword Mastery")
• ✅ Adds random skill helpers (by category / rarity)
• ✅ Adds clean instance creation so templates stay frozen, instances stay mutable
• ✅ Centralized rarity validation (NormalizeRarity, IsValidRarity)
• ✅ Better comments, safety checks, and small quality-of-life improvements

lua
--===============================================================
-- SkillData.lua (Improved, Safe & Fully Working)
--===============================================================
--  ✅ Backwards compatible with your original API:
--      - SkillData.Combat (array of skill templates)
--      - SkillData.__GenerateStats(rarity)
--
--  🔹 New goodies:
--      - SkillData.GetSkillByName(name)
--      - SkillData.GetRandomSkill(categoryName?, rarity?)
--      - SkillData.GetAllSkills()
--      - SkillData.CreateSkillInstance(template, overrides?)
--      - SkillData.NormalizeRarity(rarity)
--      - SkillData.IsValidRarity(rarity)
--
--  Templates are frozen read-only.
--  Runtime skill instances created via CreateSkillInstance are mutable.
--===============================================================

local DEFAULT_RARITY = "Common"
local DEFAULT_LEVEL  = 1

local VALID_RARITIES = {
	Common    = true,
	Rare      = true,
	Epic      = true,
	Legendary = true,
}

local STAT_LIST = { "STR", "END", "SPD", "VIT", "DEF" }

-- Rarity stat ranges (DO NOT freeze these)
local RARITY_STAT_RANGES = {
	Common    = { pos = {  1,  3 }, neg = { -3, -1 } },
	Rare      = { pos = {  3,  6 }, neg = { -2, -1 } },
	Epic      = { pos = {  6, 10 }, neg = { -1, -1 } },
	Legendary = { pos = { 10, 15 }, neg = {  0,  0 } },
}

----------------------------------------------------------------
-- Utility helpers
----------------------------------------------------------------

local function normalizeRarity(rarity)
	if type(rarity) == "string" and VALID_RARITIES[rarity] then
		return rarity
	end
	return DEFAULT_RARITY
end

local function makeIdFromName(name)
	-- "Sword Mastery" -> "sword_mastery"
	name = tostring(name or "Unnamed Skill")
	local id = string.lower(name)
	id = id:gsub("%s+", "_")
	id = id:gsub("[^%w_]", "") -- only [a-z0-9_]
	return id
end

----------------------------------------------------------------
-- Generate random stats (never frozen)
----------------------------------------------------------------
local function generateStats(rarity)
	rarity = normalizeRarity(rarity)

	local range = RARITY_STAT_RANGES[rarity]
	if not range then
		return {}
	end

	if #STAT_LIST < 2 then
		return {}
	end

	-- Pick two distinct stats by index
	local posIndex = math.random(1, #STAT_LIST)
	local negIndex = posIndex

	while negIndex == posIndex do
		negIndex = math.random(1, #STAT_LIST)
	end

	local posStat = STAT_LIST[posIndex]
	local negStat = STAT_LIST[negIndex]

	-- SAFETY: make sure ranges exist and are valid numbers
	local p1 = range.pos[1] or 0
	local p2 = range.pos[2] or p1

	local n1 = range.neg[1] or 0
	local n2 = range.neg[2] or n1

	if p2 < p1 then p2 = p1 end
	if n2 < n1 then n2 = n1 end

	local stats = {}
	stats[posStat] = math.random(p1, p2)
	stats[negStat] = math.random(n1, n2)

	return stats
end

----------------------------------------------------------------
-- Skill template constructor (frozen)
----------------------------------------------------------------
local function newSkill(name, category, description, rarity, icon)
	local normalizedRarity = normalizeRarity(rarity)

	local skill = {
		-- Human-facing
		Name        = name or "Unnamed Skill",
		Description = description or "",
		Category    = category or "General",

		-- Canonical id for lookups
		Id          = makeIdFromName(name),

		-- Rarity / level
		Rarity      = normalizedRarity,   -- current rarity of the template
		BaseRarity  = normalizedRarity,   -- original rarity for reference
		Level       = DEFAULT_LEVEL,

		-- Visual
		Icon        = icon or "",

		-- NOTE:
		--   For templates, Stats stays nil.
		--   Runtime instances get their own Stats table.
		Stats       = nil,
	}

	return skill
end

----------------------------------------------------------------
-- Skill database
----------------------------------------------------------------

local SkillData = {
	-- Expose core constants for debugging/tools if you want them
	DEFAULT_RARITY       = DEFAULT_RARITY,
	DEFAULT_LEVEL        = DEFAULT_LEVEL,
	VALID_RARITIES       = VALID_RARITIES,
	STAT_LIST            = STAT_LIST,
	RARITY_STAT_RANGES   = RARITY_STAT_RANGES,

	-- Kept for backwards compatibility
	__GenerateStats      = generateStats,
}

-- Main skill category: Combat
SkillData.Combat = {
	newSkill("Sword Mastery",       "Weapon", "Increases sword damage.",                         "Common"),
	newSkill("Bow Mastery",         "Weapon", "Improves bow accuracy.",                          "Rare"),
	newSkill("Dual Wield",          "Weapon", "Use two one-handed swords.",                      "Rare"),
	newSkill("Poison Strike",       "Weapon", "Applies poison over time.",                       "Common"),
	newSkill("Critical Strike",     "Weapon", "Improves critical hit chance.",                   "Rare"),
	newSkill("Rapid Slash",         "Weapon", "Several fast sword attacks.",                     "Rare"),
	newSkill("Heavy Blow",          "Weapon", "A strong, slow attack.",                          "Common"),
	newSkill("Whirlwind",           "Weapon", "Spin attack hitting all nearby enemies.",         "Common"),
	newSkill("Quick Draw",          "Weapon", "Draw your bow faster.",                           "Common"),
	newSkill("Twin Blade Technique","Weapon", "Enhanced dual-warblade ability.",                 "Rare"),

	newSkill("Overhead Cleave",     "Weapon", "Massive vertical strike.",                        "Legendary"),
	newSkill("Twin Fang Combo",     "Weapon", "Fast double-sword combo.",                        "Legendary"),
	newSkill("Rain of Arrows",      "Weapon", "Large AoE arrow volley.",                         "Legendary"),
	newSkill("Crushing Strike",     "Weapon", "Heavy armor-breaking blow.",                      "Legendary"),
	newSkill("Perfect Counter",     "Weapon", "Full counter after perfect timing.",              "Legendary"),

	newSkill("Ultimate Slash",      "Weapon", "Massive single-target burst.",                    "Epic"),
	newSkill("Arrow Storm",         "Weapon", "Rain arrows across a large area.",                "Epic"),
	newSkill("Dragon Slayer",       "Weapon", "Bonus damage vs bosses.",                         "Epic"),
}

----------------------------------------------------------------
-- Indexing & helpers
----------------------------------------------------------------

-- Internal name lookup (lowercased name -> template)
local nameIndex = {}

local function indexCategory(categoryName, categoryTable)
	for i = 1, #categoryTable do
		local skill = categoryTable[i]
		if type(skill) == "table" and skill.Name then
			local key = string.lower(skill.Name)
			if nameIndex[key] then
				warn(("Duplicate skill name '%s' in category '%s'"):format(skill.Name, categoryName))
			else
				nameIndex[key] = skill
			end
		end
	end
end

-- Build lookup indices (extend here when you add more categories)
indexCategory("Combat", SkillData.Combat)

----------------------------------------------------------------
-- Public API functions
----------------------------------------------------------------

function SkillData.NormalizeRarity(rarity)
	return normalizeRarity(rarity)
end

function SkillData.IsValidRarity(rarity)
	return VALID_RARITIES[rarity] == true
end

-- Get a skill template by its display name (case-insensitive)
function SkillData.GetSkillByName(name)
	if type(name) ~= "string" then
		return nil
	end
	return nameIndex[string.lower(name)]
end

-- Get all skill templates flattened into a single array
function SkillData.GetAllSkills()
	local all = {}

	for key, value in pairs(SkillData) do
		if type(value) == "table" and value ~= VALID_RARITIES and value ~= RARITY_STAT_RANGES then
			-- Heuristic: treat tables whose first element is a skill as a skill list
			if type(value[1]) == "table" and value[1].Name ~= nil then
				for i = 1, #value do
					all[#all + 1] = value[i]
				end
			end
		end
	end

	return all
end

-- Get a random skill template, optionally filtered by category and/or rarity
--  categoryName: "Combat" or nil for "any category"
--  rarity: "Common"/"Rare"/"Epic"/"Legendary" or nil for "any rarity"
function SkillData.GetRandomSkill(categoryName, rarity)
	local pool

	if type(categoryName) == "string" and type(SkillData[categoryName]) == "table" then
		pool = SkillData[categoryName]
	else
		pool = SkillData.GetAllSkills()
	end

	if not pool or #pool == 0 then
		return nil
	end

	-- Optional rarity filter
	if rarity ~= nil then
		rarity = normalizeRarity(rarity)
		local filtered = {}

		for i = 1, #pool do
			local skill = pool[i]
			if skill.Rarity == rarity then
				filtered[#filtered + 1] = skill
			end
		end

		if #filtered > 0 then
			pool = filtered
		end
	end

	if #pool == 0 then
		return nil
	end

	return pool[math.random(1, #pool)]
end

-- Create a mutable runtime instance of a skill template.
--  template : a skill from SkillData.Combat / GetSkillByName / etc.
--  overrides: {
--      Rarity   = string?,
--      Level    = number?,
--      Stats    = table? (use your own instead of random),
--  }
function SkillData.CreateSkillInstance(template, overrides)
	if type(template) ~= "table" then
		error("CreateSkillInstance expects a skill template table", 2)
	end

	overrides = overrides or {}

	local rarity = normalizeRarity(overrides.Rarity or template.Rarity)
	local level = tonumber(overrides.Level or template.Level or DEFAULT_LEVEL) or DEFAULT_LEVEL
	if level < 1 then
		level = 1
	end

	local instance = {
		-- Template reference for later
		Template    = template,

		-- Basic info
		Name        = template.Name,
		Id          = template.Id,
		Description = template.Description,
		Category    = template.Category,

		-- Gameplay
		Rarity      = rarity,
		Level       = level,

		-- Visual
		Icon        = template.Icon,

		-- Instance-specific stats
		Stats       = overrides.Stats or generateStats(rarity),
	}

	-- IMPORTANT: this instance is intentionally NOT frozen.
	return instance
end

----------------------------------------------------------------
-- Freezing (templates only, never stats tables)
----------------------------------------------------------------
if table.freeze then
	-- Freeze top-level SkillData table
	table.freeze(SkillData)

	-- Freeze individual skill templates ONLY (not instance data)
	for key, list in pairs(SkillData) do
		if type(list) == "table" then
			-- Only treat arrays of skill templates as "lists"
			if type(list[1]) == "table" and list[1].Name ~= nil then
				for i = 1, #list do
					local skill = list[i]
					if type(skill) == "table" then
						table.freeze(skill) -- safe: no stat tables here
					end
				end
			end
		end
	end
end

return SkillData

If you want, next step I can:

• Show example usage (creating instances per player, saving stats, etc.)
• Help you plug this into your existing systems (inventory, hotbar, skill trees).