Balancing level twenty unlockable weapons is a NIGHTMARE of a task. Spare yourselves the troubles, devs. Rather than unlocking a level 20 "prestige" weapon, allow players to unlock level 20 cosmetic skins for their mechs instead. That way players are still rewarded for grinding all twenty levels, but not in a fashion that imbalances the game. By providing cosmetic upgrades at level twenty to a mech-specific chassis or camouflage, you still provide players with an incentive to reach level 20 and feed the show-off in all of us, without causing odd balance issues like the Point-D Vulcan on the Assault mech, which we players can't even test properly without grinding to that unlockable point.
This can be executed two ways: By skinning the mech chassis itself, or by skinning the weapons individually with specific colouration. Think of the "golden gun" mechanic implemented in some games. I'm sure "prestige" skins for mechs and/or weapons will be very popular.
In short: Mech-specific cosmetic unlocks at level 20 only. No new weapons. No imbalance. No headache. Everyone wins!
Edited by Conquistador, November 26 2012 - 10:16 PM.