It was inevitable that some characters were going to get shortchanged given the sheer amount of territory, plot threads, and so forth this movie had to juggle. As much as I would enjoy a 20 hour Avengers movie finale in which every single character got to have their own mini-movie worth of development, that was just never going to be feasible.

By the way, it could've gotten way worse, at least none of them got the cry-baby Thor treatment.

The problem is there really isn't a better happy ending for Steve than this. Maybe he took some liberties with the Time and Reality gems to make it happen before putting them back, and maybe they all are already living in a better world that Steve shaped. Who knows?

The only problem I do have with the ending that the author somewhat got to, is if Steve knew he was going to do what he did ahead of time, he should've taken Bucky with him.

For those wondering, this is not a follow up or a sequel, this is a stripped down port of the PC game that has been renamed. There's a FAQ in their forums located here that details what is in the game.