I was thinking about Infinity War the other day and we know that Thanos is the big bad, and I thought wouldn't it be cool if Red Skull was a twist bad guy. We see him disappear but not necessarily die at the end of Cap 1 and the infinity stones are going to get messed around with, so why couldn't he be trapped in the Tesseract and be released and empowered. Rather than an external threat being the finale a very human one and one of the oldest villains in the MCU.
I'm aware that early scripts for Avengers had Red Skull as a bad guy but this was dropped, given that he represents most of what is bad about humanity wouldn't he make a great villain, just when you think you've won there is a final challenge with a much more human dimension.