I honestly go back and forth about what I most want in this game, and Day/Night is at the 1st or 2nd place slots. It's hard to decide.
1. Option for actual, balanced 5e gameplay
2. Day/Night
3. Party of 6
4. Better camping and long rest mechanics
But not necessarily in that order.
Night play would be so fun and would offer a truly fun ambience. I imagine the surface with Underdark lighting, sneaking up on enemies with advantage as a rogue or ranger, etc. Sigh. It would be so awesome.
As much as I push for more 5e, I actually think it's number 3. I think Day/Night is 1, Party of 6 is 2, 5e balanced gameplay is a close 3, and then Better camping and rest mechanics.
2 and 3, though, go hand in hand in my opinion. The game could be balanced with minimal work with party of 6 and 5e stats and rules.