I was thinking about the problems of xp debt that some people encounter, and some don't.
Really, it comes down to, in most missions, knowing the "tricks". Once you know them, then a lot of missions get super easy. I don't want to encourage bad play, nor relying on tricks. Jacking up the XP would really serve to encourage both, and not the fun and challenge level.
I was thinking about the way that Dungeon Master's guide handles XP. There's CR and all that, but there also seems to be a part about how many resources are consumed. This includes things like hitpoints, spell points, potions, wands, etc.
It seems like a decent way to (carefully) approach it would be to take these into account. I know there might be a lot of room for exploiting (going into a quest, "drinking 100k plat worth of potions and then finishing it to earn a huge bonus" come to mind), but I think there is enough benefit to encourage challenging game play to make the idea at least worthwhile to discuss, if not implement.
The greatest difficulty I see is that challenge is entirely subjective. Is going into a mission with 3 bards and 3 rogues challenging, or are they dealing so much damage with warchanters, fascinate, charms, and sneak-attack that the mission was trivial? Things like "damage dealt", "damage received", "damage healed", "spell points consumed", "deaths" all factor in, among others.
So, any thoughts on what would be good indicators of "mission challenge"? After all, we get our greatest experience from our most significant challenges in real life. Perhaps it would create a tradeoff in regards to barreling through quests to get to the end. The benefit to doing so is you use less resources, but you get a slightly reduced XP as it wasn't a challenge, whereas those that put themselves into difficult positions earn the greatest personal growth (XP).