I know the LuaJIT maintainer(s?) will never add it because it's too radical a departure from Lua but I wish they would include Luau's type annotations. There are typed languages like Teal that will compile to Lua that should work (although I've had a difficult time getting Teal to work with cdefs) and you can kind of fake it by using C structs obviously but having such a feature be native to Lua itself would be nice.