Well if you want to see a sim which offers the most to modders besides just skins and missions, try CFS3. There are 2 great new projects in the works; MAW at
http://www.medairwar.com/index.html and OFF at
http://off.oldbrowndog.net/. Meanwhile some of us are trying to recreate in detail the Bomber Command/Nachtjagd War; still in its preliminary stage.