I think 'fun' for me is having lots of different activities that dont give me time to get bored, and vary from eachother so i dont feel like im doing the same thing all the time. I like to be able to do things alone, with a friend or with a whole large group, and to be able to do any of them when i feel like it. Stuff i would get a kick out of;

1. Being able to use our path companions and level them up, change their gear appearance. (loved it in xi)
2. Large scale, open world assaults that everyone can join in.
3. Something like Pankration, because it was just awesome for those times when a few of you had nothing to do and some time to kill.
4. An option for instanced NM leves at all sorts of level ranges instead of mass running/killing leves. For when you just want a good old fashioned boss fight, not for the rewards but for the fight itself.

For me thats a wide range of things you can do with all sorts of sized groups at all sorts of times, without having to wait til everyone hits 45+. I have fun with the game as it is now but id be lying if i said there wasnt a lot of down time.