The key is to make games that programs cant beat. Turn based strategy is ideal. Pacman can be beat by a bot and well as tower defense. I have a few in mind that can't, what do you guys think?
Go and Chess would be good candidates for games with a very high branching factor (bot resistant) and you have the advantage that people are already familiar with them.
The hardest part is getting enough traffic to make PvP viable. You'll definitely need to be able to the play against the CPU to keep people on the site long enough to encounter another human player.
I wish you all the best, it's a tough job you're embarking on
Cheers, Paul.