One of the best features of JS/TS ecosystem is the ability to create and reuse code with npm packages.
Currently it's impossible to create consumable libraries for Lua.
While working on my transpiler I have investigated it, so that's what I came up with:

Variant 1 - Transpilation done by application developer

• Enforces some strict project structure for library authors
• Requires reimpementing some TypeScript's project organization features, like project references
• Makes transpiler API less flexible, since new TS programs would be created for each library internally
• Would make transpilation a lot slower, unless some caching would be added

Variant 2 - Transpilation done by library developer

• TypeScript and Babel are doing this
• Allows to create projects that compile to both Lua and JS
• Allows library author to use different/patched version of tstl and use custom plugins
• Allows library author to customize build phase as they want (cli, gulp, custom scripts)
• Allows to use libraries written manually with Lua and .d.ts files
• Requires development of standard library as a separate package (maybe in a monorepo), so multiple libraries would use only one instance and version conflicts would be avoided with semver. TS is doing the same thing with tslib.
• Complicates import path resolving process (but also makes it more flexible)
• Not allows to configure optimizations globablly. For example if Add custom Array type to lualib #262 would be done as an option it won't be possible to use multiple libraries that are using different configurations
• Libraries would be distributed like in TS - with a .lua and .d.ts files. To generate declarations tsc --emitDeclarationOnly could be used, but...
• Some of semantics is lost on compilation phase. For example

export class A {
  get foo() {
    return 1;
  }
}

export declare class A {
    readonly foo: number;
}

So that would require customizing declaration generation to insert some metadata, similarity to what Kotlin is doing with generated java class files.

/** @TypeScriptToLuaMetadata {getters:["A.foo"]} */

export declare class A {
    readonly foo: number;
}

I personally prefer second variant, but it comes with more issues to solve.