I'd love to see Crossbones paired up with Skully, and Johan definitely needs to rejoin Cap in the modern age, as he did in the comics; but from a cinematic standpoint, it might be too re-hashy to bring the Skull back immediately. What I'd prefer to see is another Cap iconic villain like Zemo or Madam Hydra, have them create a new Hydra in the 21st century dedicated to the old "ideals" of the WWII outfit, and have them somehow summon Skully back from the cosmic void at the end of Cap 2. *Then* see Skull and Crossbones take center stage in Cap 3.
THE COTTON AVENGERS
...They move like slick cotton on oil.