Made my mind up, I'm definitely going to have a crack at Downfall (I've already started doing some test graphics). In the end it was either that or a multiplayer Balloon Fight-style game - a tough call.
Thank you to everyone for your help and I may revisit this thread in future if I need any more ideas
I'll put up a new thread detailing the game's development at a suitable time... in addition I'll be posting screenshots, WIP versions and probably source code too for the curious.