A platformer or TBS game. They're not very difficult to make technically, and they have lots of fans. They could also hone their skills and get noticed by making mods for open-source video games. Quake, Battle for Wesnoth, etc. Maybe they could end up like the people who made "Overdose". They were just some guys who made a mod for a game called "Painkiller", and it was so good that the company that made the game contacted them and published their mod as a stand-alone expansion, and now they're a legitimate company.