I'm not sure that my strongest desires would be viewed as being recognizably D&D, but I believe that a few could be easily implemented.
The simple changes
~more intuitive book layout and better indexing of content
~softcover options for books
More in-depth changes
~"bounded accuracy" which actually means bounded accuracy (hard to explain, but I've found that 5e's goal of keeping numbers manageable and meaningful doesn't always work -at least not in the context of what I thought those words would mean)
~a playstyle and worldbuilding baseline which has more grit and takes a little more inspiration from older fantasy and sword & sorcery, as opposed to a baseline which assumes everything is infused with magic; a race to save the multiverse as a common idea... I guess maybe that's old fashioned, but (and it's an odd thing to say) D&D sometimes doesn't feel like "fantasy" to me, especially not in the context of things like Game of Thrones, Conan, and etc. I'm not saying those need to be modeled perfectly (and I'm sure others have different influences), but it always seems strange to me that trying to tell stories from the things I've been inspired by (and in some cases inspired D&D) is such a struggle using the D&D game.
~despite my previous comment, one of the things I enjoyed from 4E was the mentality behind encounter design and having more moving parts rather than more static positions. If there's a way to have that style of encounter design mixed with a somewhat more grounded baseline, I would enjoy it. Truth be told, I still don't really understand the mentality behind 5e encounter design.
...I suppose it's a little like 80s fantasy movies: by today's standards, the stories and effects are cheesy, but there's something about the "feel" of those movies which is rarely captured in newer stuff. I think part of that is a combination of fantastic thinking for the storylines governed by needing to use practical effects. I still want cool characters and flashy powers, but sometimes there's a realness to the guy in a rubber suit and practical effects that a digitized scene and greenscreen (despite being better) somehow fails to capture.
~more decision-making points for building a character and for playing the game... I can appreciate the streamlined appeal of a game, but I also feel that a lot of depth was sacrificed. I don't want PF2's feat bananza, but something a little more than what 5e currently offers would be nice.
For the most part, I enjoy 5e. However, I've also noticed that I'm less invested in the brand than I have been previously.