This is probably a bit TL;DR but I’ll post anyway because I’m interested even if no one else is 
I’m unsure how mission levels are going to work when you choose to have “relative levelling” turned on in co-op. I don’t know if that’s the right description but I mean the ability for a L10 and a L15 character, say, to play together seamlessly.
The only co-op game I’ve played that uses this “relative levelling” system is Dead Island. That game gives each mission a difficulty rating rather than an absolute level. So, if a mission is rated “Hard” it will be hard relative to each player’s level. Give a mission an absolute level and you get problems.
As far as I can tell, from the BL3 gameplay I’ve watched, missions have levels in the traditional Borderlands way. Normally, that would mean you could go into a mission over-levelled and have an easier time than being on or under-levelled. How does that make sense when the game adjusts itself to the level of the characters playing?
For instance let’s take a couple of players, A and B. A has blasted through the main missions only and is L10. B has reached the same point in the game but has done all the side missions and is L12. The next mission is L11. A should find it tough but B should find it easier.
But what happens when they get together in co-op? The “relative levelling” should mean that they are doing similar amounts of damage relatively (obviously!). However, if A hosts, the mission will be tough for them, so will B find the mission harder? Conversely, if B hosts, will it make it easier for A? If this adjustment doesn’t happen, then B will be doing more damage than A, which is exactly what is trying to be avoided.
As an adjunct to that, how is enemy XP decided? Will enemy levels, and, hence, their XP value, be dictated by mission level, as is usual in Borderlands? If a L5 player joined a L20 player doing a L20 mission, the game should balance the relative damage they do but will the L5 player get the L20 XP for enemy kills? If so, that’s a “good” way to level up quickly.
As a bit of an aside, while checking this out, I did notice that rewards are no longer listed on the mission “cards”. This does leave open the possibility of variable XP rewards according to character level. Or, perhaps, none at all, with the mission gameplay providing the reward.
Does anyone know how this is going to work? I suspect not, except for Gearbox
I’ll be interested to see their solution.