I always kind of liked the AI and Research levels in Anarchy Online.

At any point while leveling or at max level, you could either go kill a lot of aliens for AI, or allocate a percentage of your normal XP to Research. As these gained levels, you got Perks; essentially a kind of skill point you could put in, for instance, getting +10 to all stats (would probably be more like +1 here, as AO left you at something like 1500 in each stat at max level), increasing specific weapon skills, getting better evades or tank skills and so on and so forth.

The point was that you could choose to do this at any time, meaning there were points in the game when you were better off switching to Research for a little while for faster stat gains rather than forging ahead for max level.

I also miss how advanced the game's skill system was for its time, so maybe I'm just nostalgic. I don't know if I would have the patience for that anymore.