I'd say, after a certain level, XP should be more difficult to gain.
But that would mean, even centaurs would be outclassed. We need new, powerful or smarter critters. Like Enclave getting the idea of leveling up and actually using strategies, like flanking.
Even more nightmarish creatures, like Gehennas and Molachs from the east.
Gangs with CAs and plasma weapons, cyborg cousins of Frank, insane military robots, militant technophiles, splinter Enclave groups, mutant factions...