It would be cool after leveling a class to the maximum for you to receive an xp bonus, for example: 

the first class was leveled to the maximum = 0.75x xp

the second class = 1.25x xp and so on

I think it would make the game more dynamic and not necessarily easier or more 2xp events could be done

because friends of mine have lacked motivation to level up a new class because they either don't have a weapon that makes a difference (like the sniper that only have the bolt pistol in common with others classes) or because they simply don't want to go back to the easier difficulties