I've been a member for a long while and come here to learn; there are folks here with far more knowledge than I'll ever have to add. But since it seems that the devs do indeed keep an eye on the forums here, it seems worthwhile to speak up,
I'd have to nominate ATC first. While ATC in FSX/P3d may not be anywhere near perfect, it's still the best we've got going and I'd like to see at least that level of ATC in the new sim, and then build a better system on top of it.
Beyond that, I'd like to see seasons implemented ahead of VR. The completeness of all-weather flying is a key item to me.
I have to be a bit contrary about VR. I appreciate it, and I'd love to try it, but without a solid sim in the first place, VR is just eye-candy and gamelike. The MSFS series has distinguished itself a simulator first; let's not lose track of that. First a solid simulation basis, then apply good VR to get the most from it,