I was at a car boot sale yesterday. And some one was selling a box of old Amsoft games for the Amstrad CPC. I just looked through them thing "these would all be great one man projects for Blitz3D". So maybe you could try looking up old home computers, seeing what games were on them and copying some of them? Maybe add a few new features, change the levels around and obviously call it something else and you've got a whole new game you've developed all by yourself.