My wishlist is pretty simple. I haven't been able to run a version of MM since 184.108.40.2067. I actually keep that install exe file with me and have it backed up in two places.
I just tried the new 4.x version and, surprise!, it installed only partially due to an error. These errors have happened on three different computers over the years.
I have a 15K song library which grows about 1-2K per year. I listen to music off my computer, and that's it.
Your versions up to the 3.x version noted worked great. I love the software. But, as technology advances, my guess is this older version will - some day - stop being viable (opertating system driven).
My wish is easy: keep in development a simple stripped-down version of your product. It's all the new/extra stuff which seems to cause the problems (an assumption, I guess). I don't need it to connect/sync with other devices. i don't need it to manage videos. I hate that I always have to deselect the shopping function on reinstall.
The track indexing to quickly filter and sort in a split-second is excellent. I "sell" you to anyone who will listen. Just keep a simple version. That's it.