We still have a lot of people with no avatars set and stuff like that. There are two ways to get people do things they are either lazy to do, or just don't care.
One is coercion, the other is incentives
I'd stick with the latter, was thinking that they could get some rewards for certain actions (of course this would apply to new players too), for example setting their avatar could get you some currency and some minimal stats (like minimal, but something), then if they link their forum account they would get something too, and actually these could all be part of the process when someone creates a new account.
Imagine it as part of the tutorial. (Idk if there is a tutorial at all, it would be a miracle if I remembered)