I think the one thing I would change is, while probably a bit broad, it would be a return to having the highest quality of everything
I recall when I was a kid my mom's # 1 comment about Walt Disney World was that "it is so clean!" - and just knowing how Walt was about the parks, etc. ... just would like there to never be non-working animatronics or even small things like missing ceiling tiles on It's a Small World, would love for Traditions to go back to being 3 days and more intensive, etc.
I am not as down on things as I hear some complaints and stuff but from my experience, I know it can be even better and that is what always set Disney apart - so it would be a return to that - no short cuts, no full garbage cans, etc.