I realize my suggested player goals are biased by what I find fun. I've always defaulted to the mage seeking more power through knowledge of spells and rituals. But I think if you leave the categories broad enough that players will find room to pursue and be rewarded for goals they enjoy. So here is some rumination on some more possibilities.
Maybe you like figuring out who is pulling which strings and ultimately you want to pull those strings yourself. I could see this working: finding out information could provide rewards that are common, uncommon and rare based on how big a scoop you discovered. And I suppose these power seekers would receive rewards for attaining levels of power: you found an informant, common, you've gained entry to a guild, uncommon, you are a guild leader, rare.
Mixing the knowledge goals and achievement of power goals muddies things for me and starts getting a little to fiddly/specific. Also, the way oe gave less XP for magic items because they are their own reward, isn't achieving power its own reward? I think a player really interested in climbing in political power could work within our simple Knowledge Seeker category. How they put the knowledge to use is up to them.
Another potential problem with rewarding political goal gains is that it seems more solo oriented. What is my artifact hunter going to be doing while you are trawling the court hangers-on for juicy gossip?
What about the classic tale of a member of a disgraced clan trying to clear the family name or get revenge on the powers that brought the family low? I think there is room in the 5 categories for this. Maybe the magic sword passed down from generation to generation is a rare artifact the player is hunting for. Maybe the knowledge of the whereabouts of the nemesis is an uncommon XP goal. I suppose you could give XP for achievement of name-clearing or raising the clan status again but I'm not sure how I would measure that as a DM. Rewards for the goals leading up to revenge/justice seem clearer.
I have similar reservations about this as the last category though, 1) you have mixed goals, knowledge/artifacts/achievements, making everything more complicated, and 2) you have a very personal set of goals, why would me mage interested in spell-seeking want to hed into the black swamps to find your father's killer?
Feats of Daring-do/Glory!
I you are a warrior at heart, perhaps you won't be so interested in the goal of protecting your fellow adventurers. You really want to bring acclaim to your name by feats of your sword arm. You want renown for facing off against multiple foes etc. But what would the levels of reward be? I imagine uncommon would equate to either fighting multiple foes at once or a foe of great prowess. And then rare would be twice that? So to get rare XP rewards your first leveler is fighting a minotaur single-handedly or likewise? I don't like the sound of this. Seems like a reward system meant to increase your player death toll.
Let me drop the possible new categories here and think about reward categories in general. I realized a few things as I was typing the above paragraphs:
A reward system only works well if it is understood by all parties.
In other words, you want players and DMs to be on the same page. This is easy with treasure rewards; mo' booty means mo' XP (and mo' problems, ha!), less so with XP for killing creatures; "How many XP is a goblin shaman worth? should we go back and kill it or just lug this treasure out?", and especially difficult with XP rewards for roleplaying; "Does the DM think my dwarf would chase the bandits or stay to loot the corpses? Does she want me to say something witty or be dour/taciturn because that's what dwarves in her world are?".
Broadly defined reward categories allow for awards arising randomly.
See, the good thing about having broad goals is that they can come from random rolls. Common, uncommon, and rare artifacts are only a chart roll away. As a DM you can specify the likelyhood of an uncommon result in a certain type of locale but you aren't required to place items in advance. In fact the risk of the opposite (and this is true of the categories I was pondering above) is that it becomes story-driven and railroady. Rather than a party of adventurers heading into a crypt for loosely related purposes, your character has to twist everyone's arm to go into the Lich Queen's lair because that is where your ancestral blade is.
Can't these two rules-of-thumb contradict each other, though? If a reward system works better when I know exactly what rewards I get for which actions, how do I know how common something is in your world? I think this is a possible weakness and strength of broadly defined reward goals at the same time. On the one hand, players won't know how common finding Gauntlets of Ogre Power will be. So when they pull it from a chest they might be thinking "Is this good, or really, really good?" But on the other hand, a cool aspect of rewarding based on frequency is that players will eventually come to know exactly what is common and what isn't. "We have only ever found Gauntlets of Ogre Power that one time." Hmm, maybe the last is too obvious to really be important, but it seems that reward knowledge would be perfectly transparent if you are rewarding on the first sighting of a creature and that players will realize the second sighting is less significant. Which leads me to think:
Rewards based on rarity will push players to explore.
Right? If you've observed goblins and rats and orcs, or you've found all the first and second level spells twice over, you'll want to go over the next hill or push ever deeper into the megadungeon.
Okay enough blathering from me. What say you?