I thought this might be interesting, I'm hoping to see a range in answers. Adding on a 'why' would be great if you can! In order, preferably.
1. Mirrors! I need mirrors in the arenas. I know my riding would improve so much in just a few short weeks if I had mirrors, since I'm tackling an issue of tenseness in my arms. I just need to get it into my Muscle memory and then I'll be fine, but when I'm riding I'm so focused that I forget to remind myself of my position. I rode in an arena with mirrors a few times and loved it.
2. Bigger turn out pen(s). My horse just needs more space and more time out than he gets at the barn he's at, and right now I have to juggle between turning out in the outdoor arena or the mediocre turn out spaces.
3. Grass pen, so he can eat grass while I clean his stall instead of having to hand graze.
4. A dressage simulator, for my position and fun.
5. A rainbow of dressage saddle pads. That would just brighten up my day, and then I could really see what colors he does best in.