They should make the Max level 35. it's already insanely long to get to. Most of the highest players are 33 34 very very few 35 or so.
So make max level 35. Increase Xp gains per tick, make every tick every 30 seconds because some rounds end very fast and you barely get anything, if you have x5 or high mulits. (This means halfing the Xp, instead of 1000 per minute at times 1 it's 500 every 30 seconds) You should be rewarded with slighty more Exp if you help the team out more. It will still take a VERY long time to get to 35. This bonus below is just a helping hand, an increase to make it slightly faster for players who deserve it.
1. Maybe plus 30 xp per kill or something, a small number but it helps or maybe slighty more. bonus exp for opening gates, being there to cap a flag or save your flag. Stuff like this. And also losing a little exp for teamkills. All these are good incentives to win and have fun while helping your team.
2. Many players who do the bulk of the work; by kills, capping the flags etc should be rewarded with extra things. Vsers the people that just leech, or don't assist that much at all. It's simple, they'll still get the normal exp everyone gets. Plus the Xp from the kills they do get.
This will just reward the players that put more time into the game. Vsers the players that sit there with their thumbs up their asses leeching being useless. They shouldn't get the same Xp just cause some players on the team are carrying them. If they want the added xp they work for it.
this would be a fair system making everyone have reasons to help more.