a lot of the one shots have dealt with something from the previous movie.
So keeping that trend, the One Shot added to Cap 2's DVD release may be something relating to Thor 3.
possibly Loki on the throne. or Lady Sif and the Warriors 3 crusading the 9 realms.
Alternatively, it could be SHIELD/HYDRA related or even GotG related.
Hopefully it's a Hawkguy short, since I guess he wasn't in the Cap 2 movie. It can cover what he was doing while the whole movie was taking place. Maybe he was on administrative leave, or vacation time or something. It should be a 15 minute short for "A Day in the Life of Hawkguy" sort of hyping him up for Avengers 2, and testing the waters for a limited short series starring Hawkguy.
a series of 15-20 minutes shorts covering the adventures of Hawkguy (guess starring appearances including Black Widow, Fury, Hawkgirl, and whoever else is in that series.) to be an additional extra on the Marvel DVD's along side the now traditional "One-Shot".
I'm sure Renner would be more than up for something like that.