Here's a few ideas...
Imagine if Asteroids were an adventure game. Same gameplay, but add the space-trading and exploration element. Check out Gate 88, then take it a step beyond.
I'd also love to see something like a Wing Commander, but again, more open for exploration. Even if the starter kit is mostly smaller maps with dogfights in mind, expanded universes aren't too far off. Vega Strike is a good engine, but I think that something similar for XNA with simplified controls would be killer.
I wouldn't mind seeing a tactical RPG kit. Maybe isometric.
While I would like to see an FPS kit, I think that an Elder Scrolls style kit would be more useful. Using such a kit as the basis for an FPS would allow for a richer game than just running and shooting.
Lastly, I would really like to see something like a "Metroidvania" kit. I know it's just a matter of modifying a sidescroller, but it's different enough. There has to be map transitioning, items and upgrades, maybe even NPC interaction.