This should never happen. You don't force extra steps on brand new users. That's just asking them to say "to hell with it" and sign up elsewhere. The only people who actually care about avatars are the people who post on the forums... You guys. The majority of new users only care about the game, and not their profile.
You know, I've worked with vBulletin and Xenforo, and pretty sure with those you can tie the default avatar into postcount, account age etc, so should be the same on this one. IF you said that it would be nice to see it change with a user's post count as a status thing, that could possibly work in making the forums more active. However, that should still be overwritten when someone adds their own custom avatar.
Things like that need to be as unintrusive as possible.
