Make a sandbox game of some sort. Like Gmod a little bit, but not the same.. Just.. I dont know. Make a sandbox game with the ability to build stuff and spawn weapons, and kill people by turning killing on and off, rather than a mini-game like Blockland. Have it a little more detailed than Blockland, but not 'realistic' like the stuff on Gmod, CS:S, DoD, etc. on Steam.
If you go on with this, ask me for suggestions, and I'll help you think of stuff.
Also, no matter what you do, I'd like to beta test. (if that's ok.)