Walker would be my number one choice of course. And if they weren't jumping right into Infinity War now would have been the perfect time to bring him in. Cap gives up his shield, goes AWOL in Wakanda, Secretary Ross picks a temperamental replacement that he thinks he can control and everything goes to hell. Sigh...
I am a little bummed that Crossbones is gone for good but I know there are only so many villain arcs they can include in these "Phases," especially with a number of the main actors assumedly coming to the end of their tenures. As it stands Crossbones is the only Mark Gruenwald character to ever make it to the big screen. It would have been neat if the the vial he stole in the opening of CW was called the "Bloodstone virus" or something just for fun.
At the end of the day Marvel Studios/Russo Bros took a crappy comic and made it into a great film. I'm sure if they made an awesome Cap Wolf movie there'd be young folks complaining that it didn't end with him announcing his allegiance to HYDRA. Since, you know, bad comic stories are supposed to be combined in order to be "done right."
Especially if it was a Kirk/Spock Wrath of Khan moment between him and Tony when he finally goes. I'm gettin' misty thinking about it now.
MAN would he be missed by so many other characters though (unlike a certain other recently expired hero.) Bucky, Sam, Widow, Sharon, all the people who attend his Smithsonian exhibit every year, etc.
The only thing that might hinder the impact would be if audiences were "deathed out" by all the fake deaths we've already seen in the MCU (and soon DCEU) plus Han Solo, etc.
I actually think that would be a great idea post-Infinity War. I think it'd be great if after the 2012-2018 "Avengers era" that once Thanos is put down they go a good 5-8 years of solo films with maybe one extra character here and there like Widow in TWS. Then they can go back to some big crossover event after characters have gotten their own individual trilogies or what have you.
I never made that particular observation but I remember people talking about it when the film was released.