They could add an arbitrary chance of failure, but doing that seems kinda silly.
No agression here, but why would it be silly ? All RPG systems I created are based on range of failure/successes, among other principles. You fail, you lose a lockpick. Critical fail might raise some kind of alarm, or destroy the lock, or whatever. Just need some imagination to make it "fun", espacially in a multiplayer RPG stuff.
The base game of Original Sin has a smaller world with fixed-level enemies that do not respawn.
Fine for the smaller world. Non respawnable enemies is an open world killer IMHO. There should be some sort of respawnable stuff, be it animals, plants, foraging enemies, and so on.
No respawn, would be terrible for the living feel of the world and the replayability. My real life would be probably more interesting than Rivellon :-(.
but adding in renewable sources of XP to the base game will screw up the balance.
I disagree. It would increase the feeling of interactivity, increase interesting world interactions, and make every little actions less boring.
A steady flow of low XPs depending of your actions and skills, and then some big one coming when you succeed at quests and whatever.
