While I think Pantheon will be built around a big world with few quests, I think OP's idea can still be implemented by granting experience for more than just killing. So rather than your quest being to go get that flower, going to get that flower turns out to give experience. Exploring a new area gives experience. Crafting a new bag gives experience.
The issue with these sorts of mechanics, however, is that some can potentially make it VERY easy to powerlevel. We would need some strong balance work, and limitations. We would probably want diminishing returns on XP you can get from each individual non-combat thing you perform.
Another idea is to overfill the world with non-combat quests acquired through the perception system, but very few actual combat quests, and balance it so that either means of leveling up (doing the non combat quests OR killing things OR doing both at the same time) is somewhat equal. But again, I don't think Pantheon is being built to allow that many quests.
There's some quote somewhere that goes something like this....
"The most valuable thing you can give is time, because you can never get it back".
Gideon said:
In some other game i won't quote, you learn to perform hang gliding by ... killing mobs with a flamethrower. When you have killed enough of them, you learn to fly with a glider. What's the relation ? None. Need to level ? Kill more. Pff.
Can you, uh, actually go ahead and say the name of this game. Just so, you know, I can avoid it? Yeah...avoid. Sounds terrible.