I'd like to add my vote for a Submarine type of game, the difficulty will be making it 'special' even if you could license the starfury engine, could you really steer a summarie from outside ? ;p
A 4x Risk Game would also be great
but again how could you make it 'special'?
Any game made in this method would HAVE to be easily modable - if not open source.
To throw a spanner into the works, how about making the game 'online'? so it could be multi player? I mean only the actual kinda engine -still have people buy the game so as to not have to compromise on art...
GreyCloud