This is basically the idea. We won't have 'better' stuff locked behind a level wall (improved weapons, better stats, &etc) but you will have ways to track and indicate your experience and progress.
We figure being a more experienced player is advantage enough.
You could make it so that the higher leveled players get priority to play the roles they want to play instead of who chooses the class first gets it.
(It might help avoid situations where you have a complete noob who got the game in a sale choose a squad leader class since the class seems to have the best weapons but the player doesn't know they can call in arty and such, but with a system like this the player will have to play a few games and they get a better grasp on how the game is meant to work)
If you want to design a level up system to reward players who play recently and constantly then design the system to decrease a players level over the amount of time they don't play the game.
It may cause complaints though for players who played the game for a month and took a month break and came back and they can not get the roles they want but maybe thats the incentive for players to want to play the game more.
But on the other hand without a system that decreases levels over time. I can see players who are new but maybe played a fair amount of time to level up would complain that the players who have played before but haven't played in a year still get role priority even though they might be rusty could be frustrating to the newer players but that might be the incentive for them to level up.
Just throwing that idea out there.