I'm not a fan of unlockables that give experienced players an even greater advantage either, but how about little extras that don't?
Imagine an achievement system similar to what Xbox Live games use where you unlock extra [high quality, developer approved] skins, colour combinations otherwise unavailable and perhaps, I don't know, level editor objects for use with your Teewars profile only? It might not be easy to implement and prevent people from getting their hands on the extras the easy way, but it could be fun for the completists among us anyway.
Of course it would depend on how challenging and interesting the achievements would be, but there's no rush; they could be added one by one as the creativity department comes up with ideas. I have a few myself already, but sadly no way of knowing how hard they'd be to detect during an online game. For now.