I have had a great number of ideas, and I've even made threads here on the escapist for a few of them. It would get very long if I described everything I've come up with for all of them, so I'll try to sum a couple up.
1. Terminator, preferably sometime after the end of the machine war + Bethesda's open world and combat system, Falloutesk but with themes more closely fitting modern times rather than that eternal 1950's thing. The idea is that the war has ended, Skynet is gone, humanity is trying to rebuild while the machines sentient and not freed from Skynet's control are trying to find their own niche in the world. The result being that both have split into 3 factions each + a psychotic faction (think this game's version of raiders) for each. The player would be a newly built terminator that starts out a cheaply made barely humanoid terminator and through experience and acquiring the right materials gradually upgrades themselves, eventually reaching the T-X. The actual story I've always drawing a blank so I'll let someone with a better imagination go from there.
2. A Dragonball game that goes from Dragonball to the end of GT, with a rather extensive customizable character creator with every major race in the series playable, a fly (walk until you learned how to fly first) around the world story mode similar to Budokai 3, and a battle system similar to Budokai Tenkaichi 3. The story mode would be from the beginning of Dragonball to the end of GT as mentioned, with a morality system, siding with the heroes, villains, or fighting everybody. The player would be able to both follow the series plot outright or screw around with it, such as replacing Goku as the main protagonist, or thwarting the heroes or even villains both at the same time to become the ultimate antagonist of the game, and so forth.
3. Quick one. A Gundam game that let's you play as a rookie pilot in the various series wars and effect the outcome of them, with fast paced combat, something like the Dynasty Warriors Gundam games but something to make the player's kill ratio a little more... believable. I don't recall even the most overpowered of protagonists killing hundreds of suits in 5 minutes, even in their best suits not to mention in the trash mob suits like you can in DWG.