You can add a title system or how do you call it that adds "stats" if your character met an amount of kills that levels up or if you don't want to add this on a custom map an alternative map, you can add this on BG and Woe.
lvl 1 title require a total of 10 kills
- total 3 hp/mp, xx hp/mp regen
lvl 2 title kills 100
- total 6 hp hp/mp, xx hp/mp regen
lvl 3 title kills 1000
- total 10 hp/mp, xx hp/mp regen
lvl 4 title kills 2000
- total 20 hp/mp, xx hp/mp regen, 1 hit rate, 1 flee
lvl 5 title kills 3000
- total xx hp/mp, xx hp/mp regen, 2 hit rate, 2 flee
lvl 6 title kills 5000
-total xx hp/mp, xx hp/mp regen, 3 hit rate, 3 flee, + 20 weight limit
increase effectiveness of healing items, aspd and so on...
Every level the total kills increased and add more stats. I chose these stats bcos I think these wont greatly broke the balance and make particular character OP
This is just an idea you can add these kind of mechanics on BG or Woe.