Well an 'add-on package' might work, but it would probably be best if it's a simple 'install and go' type of package. The idea being that it's all there ready for you to do game development. No hastles or complicated setups... Thats the idea of a Game Development Edition or Package as you suggest aswell.

But as for whats contained. I recommend ALL the APIs Headers(including DirectX because of it's popularity of use) and toss in any other cross-platform libraries. ie. JEDI-SDL as one I know off the top of my head. Or simply have SDL Headers too.

GLScene for Lazarus could also be incorporated into this aswell...