I think Expedition league is one of the most enjoyable league mechanics overall (just the league, not the changes) and I really want to see it be a part of the core game as a master mission.
"But expedition sucks look how few people played it!" you might scream internally, but I'll be honest: the core mechanic is no less disruptive to the mapping gameplay than any of the other master mission mechanics (except Niko and Einhar, but you can argue that they take up time outside of maps too), is enjoyable and gives a good chunk of mob density, and gives access to iconic parts of the ARPG genre (gambling, etc). People's issues weren't with Expedition, but with all the negative changes GGG made. Chasing and running logbooks has been the most fun part of the league for me, and I'd love to be able to do that as often as blighted maps (even if rewards were nerfed a little bit).
I think Expedition would work as a master mission league better than a random spawn is because we already have 4 different NPCs to work with; it would be really difficult to gain enough of each of their currencies to actually do something with if a) Expedition only has an 8% chance to spawn and b) from there you have a % chance for each NPC. I think each NPC does bring something valuable to the table and therefore the "crew" being masters would still work.
Again, this is all my opinion and I'd like to see what you think. Either way, I think Expedition is as a mechanic just as enjoyable and worth repeating as Alva or even Jun.
TL;DR, unpopular opinion that Expedition should be a master mission and that the game would be more fun for it
EDIT: getting a lot of comments disagreeing with my point of not being too disruptive to mapping gameplay. I totally get what y'all are saying and you're right it is a little more time and thought intensive than other mechanics, so I was a little off-base. However, I do feel that (this is 100% just my opinion) it really isn't *that* bad and the gameplay loop resulting from doing it in-map justifies it in its current form. Again, just my thoughts :)
#2: obligatory thanks for awards edit :)