Last night i has a idea about what to do after lvl 21:
SISTEM OF LEVELING GUNS AND CRITERS
CRITERS
1.- If you kill a critter certain number of times you will get a bonus against him
2.- this is like a sistem of lvls more kills more lvls more advantages
3.- advantages are little, for not kill the actual sistem
4.- is a way to grew up REALISTIC
5.- the advantages will be gived as Perks, with all his parafernalia, funny image, name and silly description.
6.- only work in PvE
Example
Mantis
kills         lvl              advantage
10           1                  none
50           2                  manti have -5% probabiilities of hit you
100         3                  you have +5% of cause a critical hit in a manti
200         4                  you receive -5% of damage from manti
400         5                  you cause +1 points of damage against the enemy (per bullet)
800         6                  you ignore all the DT of manti
1000       7                  you cause +10% of damage to manti, and Bloody Mess is enabled against them (for funny)
WEAPONS
probably the most important part, you can lvl up you hability with any weapon
1.- Every time you kill something with certain weapon the xp also go to the lvl of weapon
2.- advantages are give as perks, again with all parafernalia
3.- the advantages are only for the weapon lvled up
4.- every weapon has his only scale of lvls
example
Weapon SMG 10mm (H&K MP9)
lvl            xp needed          perk                                     advantage    
1              1000                 H&K MP9 Beginer                   none
2              3000                 H&K MP9 First Step                you get a bonus of 5% xp for normal lvling
3              5000                 H&K MP9 No Fail                   you have -10% probabilities of have a critical failure
4             10000                H&K MP9 Degeneration         your weapon degenerates 10% slower
5             20000                H&K MP9 Resistance            you ignore 5% of damage resistance from the enemy PvE
6             30000                H&K MP9 DT                      YOu ignore 1dt from the enemy
and then,
as smg 10mm is a burster weapon you will have advantages with bursts. more damage per bullet for pve, and then for pvp, at las lvl you will need like 300000 xp for "you cause +1 damage per bullet in pvp"
this way old character will have a advantage against new lvl21 characters, and is a good way to especialize in some weapons and enemies
i know: is hard to implement but i cant 

 this
sorry for my syntax, im doing the best i can