My vote would be
1). Make a way to find Pokémon. Sightings and Nearby are both failures. They work against each other and make for a broken system. I'm tired of not being able to find Pokémon.
2). Ability to request Pokestops, even if it is only available to high level players like they did with ingress something would be better than nothing. I have a list of over 20 local places that would be great fits for pokestops but alas there is no way to do anything about it.
3). Shiney pokemon release. Even if it is a limited rollout it would tide people over and keep interest going for weeks or months to come.
4). Rotate Raid Content. It would be fun to see a migration schedule just like nests. This is #4 on the list because lots of people still need some of the Pokémon currently available in raids.
5). Gen 3 Release
6). Trading. Easier to implement than PvP and I have been saving lots of good Pokémon to trade with players who are just starting out. Lots of 93-96% evolved pokemon will be up for when it comes.
7). PVP: I want this one last because Niantic needs to take their time and get it right.