This one is not very feasible.
Basically you create a weapon editor, map editor, creature behavior editor. These things are fully featured, with intuitive UI and simple to use, while also allowing access to the underlying code. You populate it with a bunch of sample 3D models for a variety of settings and boom. Include instant action, multiplayer, and single player and let the entire game sort itself out. Of course you’d also want to make an example scenario with all of your team that would sell the tools package and show off your skills, but the whole point would be the editor (see Neverwinter Nights, a truly awful game made good by user content only). I don’t think it would be that hard, if the editors were, rather than game development tools, user-oriented creation machines, to see a huge community cropping up. The entire ModDB community would be transformed overnight from a failed junkyard of mods into a frenzy of Generic Shooter scenarios that all, at the minimum, were playable. Besides, people would buy this game on name alone. And you’d get the prize of first fourth-wall-breaking game title (I think).