This is why The Bloodstone Hunt would be the perfect base/inspiration for Cap 3. Zemo would ostensibly be the Main/Major villain in the film, they should go with the son Helmut Zemo and his quest to find the parts of the bloodstone in order to resurrect his Father, the original. They could rework certain roles from the story, have the mercenaries working for him perhaps keep Batroc but add Crossbones to the mix and perhaps have Diamondback working for the baddies team but then have her switch sides, become a potential love interest for Cap to cause tension with Sharon or even just for the sake of a little variety. Maybe even work in some nods to the whole Crossbones/Diamondback "Ratsel" connection from the comics.
Then at the end they could have Crossbones betray Zemo to reveal that he was working for Hydra who use the Bloodstone to resurrect and/or bring back The Original Red Skull just in time for Avengers 3 where we can have Thanos, Loki, The "real" Mandarin, and maybe Ultron for a variation of The masters of evil. That's what I would like to see.
The writers seem to imply that the '50s pyscho Cap could potentially be the villain in Cap 3. I feel they should save Burnside for Cap 4, and maybe even make him a hybrid character combining some elements of Protocide too. He would be the Anit-Cap/Dark mirror/evil version character. Doctor Fautus is a favorite of mine and it wouldn't be too much of a stretch for him to be the mix if they go the pyscho Captain route. Maybe we could even see Jack Monroe/Nomad or even John Walker/US agent, although their not villains, still I would like to see them at some point if possible.
Flag Smasher would make a good sub villain for a future Cap movie. Maybe a villain in the opening scene or first act. He's another villain from the comics that I liked. Could tie into Red Skull's comment about the future having no flags.