My tuppence on the matter:
1) Ehhh, not a fan. Whenever I play a game and see characters accelerate through several levels in a single jump (exceptions made for getting low-level characters into a high-level party and receiving high-level XP rates), I feel like the designer has made some exceedingly questionable balancing decisions. Even with a 99-level progression, it seems entirely out of place for people to gain several levels by merely fighting a small band of squishy, common goblins. It's a worldbuilding thing, mainly, to be honest, but one that does stick out like a sore thumb to me.
2) Whether in tabletop or video games, I disfavor individual XP gain for collective efforts like combat, and per-kill or per-action XP can quickly skew progression badly, especially if you have characters dedicated to buffs and healing contrasted by characters dedicated to tanking and spanking. However, individual XP for, say, class-specific or character-specific out-of-combat actions can be appropriate, such as if the party's healer saves the life of the young prince, or the party's thief manages to disarm all the traps and pick all the locks in the villain's hideout, that sort of thing.
As for interesting/unique ways to gain XP - I see Quest XP massively underutilized in JRPGs, though it's hardly "unique" or "interesting" as such. If you want something more interesting, perhaps a "Nietzche State" where a character's XP rate is increased by the amount of HP they're missing at the end of combat?
You might be able to make something even more interesting by using XP as a point-buy system instead of a leveling system, though - perhaps something akin to the mechanics of the GURPS, Storyteller, or Fate systems for tabletop roleplaying? I made something like that for 2K once upon a time, and let me tell you, while it was technically working, it was an eventing fustercluck.
