[Snowflakian]

Characters I'd kill to see but will never be made?

Amalgam. Plain and simple it's in rights hell.
Original Death's Head would be problematic too.

Everyone else for Marvel/Hasbro is open. Circuit Breaker, Rom, and even the legions of obscure marvel characters. I mean heck, they did make a frost giant, constrictor, DarkHawk, and even had a fans choice poll. Not to mention they have plans for even more obscures down the road. It just takes time. They have to balance well-knowns against obscures to keep the waves palatable to retail outlets and kids as well as collectors.

They've also said previously in QnA's that Rom is on the table for someday, but who knows when.

Everyone else is fair game for eventually. It's just a matter of when. For example, we might see that FF Mole monster as a GBP when Fantastic 4 reborn hits theaters. IronMan 3's line may see more comic villians, as well as any other movie sequel. They have to balance what lines will get movie lines alongside what's in MU. Think of it this way. You know for fact Ironman 3, Avengers, Cap 2, Thor 2, and more X-Men/Wolverine movies are coming. So do you do all of them now in MU, and pray kids/collectors will buy the new ones for their movie lines, or do you space them out and cover things you haven't yet first. IM2 classic Ironman is a perfect example of this as a test and how he shelf warmed/needed a full on flightbase as a push.

I fully expect someday we will see all of the solely Marvel owned licenses eventually(well, barring stuff like captain Axis, and similiarly overtly nazi themed that can't be pushed into hydra or similiar), and ditto to the Hasbro ones that are in the Marvel U. Even the newer Death's Head is probably fair game. IIRC though the classic one isn't part of marvel's library of rights.

After how lowcut X-23's figure's clothes are. I wouldn't count Sue Storm as Malice out either in a Psycho-man Malice comic pack eventually.