I don't necessarily think that some things 5e did away with were all that problematic (e.g., attack rolls vs. defense).* It was a smooth and intuitive mechanic that streamlined the game and made it overall easier and quicker to run. From what I have seen here, attack rolls vs. defense rarely, if ever, seem to come up in the complaints that people have against 4e. However, the architecture of the two games diverge in some pretty key ways, so it's not as if they could be added to the game. As such, there's not too much, if any at all, that I would want from 4e that I can realistically expect will make its way into the 5e core revision without it becoming a new edition entirely.
* I have seen several people in the OSR community, for example, advocate for magic attack rolls against static saves, only for them to be shocked to realize that this is how 4e did it.