Thanks for all the replies so far. Really impressed at the quantity and diversity of the ideas. Do keep 'em coming folks.
I'll do a more in-depth reply later on to address individual suggestions but so far I've taken a real liking to Invadazoid and Downfall... the latter in particular doesn't look too complicated. But I will look more closely into both of them.
There are some suggestions I'm not too familiar with so I will be doing plenty of research methinks.
Thanks again, really appreciate the ideas coming out here!