AC is an odd one, I am more on the side that liked the historical aspects of it and am not huge on the mythology angle they have tilted to. I was never huge on the present-day sections of the series, but it is kind of the spice that gave it it's unique flavor I think.
2 was the last one I fully completed and loved it. Still have fond memories of murdering people up and down the Italian countryside, jumping into canals and haystacks, etc. The sequels felt more like expansions, and it's looked kind of cookie cutter for a long time from the sidelines. They just overmilked the series and should have had less of them. Avoided 3 due reviews and disinterest. I tried again with Black Flag but couldn't get into it. Unity, and Rogue Syndicate that were freebies at some point, but haven't touched.
Finally grabbed Origins on sale late last year due to the history tour mode, and it's really cool. Haven't had a chance to try main game. Saw Odyssey had same mode and has been getting really good word of mouth so rolled dice on the ultimate rev at end of year sale so I should be good on AC for a WHILE. It came with the remaster of part 3 too. 
Norse setting is super played out anyway, will grab down the line if they have a history mode though.
I think an AC in Tsarist Russia would be great also and of course Japan, India or China.
Aside from Japan, weren't these the settings they flirted with in the side-scrollers?
I think they should start to mix the History Tour things with the game again although it might not be what everyone wants.
Odyssey really surprised me by how good it was and how much they learned from BOTW and Witcher 3 and the likes but at times the gameplay loop becomes a grind.
In that way it has the same problem that most Ubisoft games have. They have a ton of content but too little variety.
It also depends on your play style I guess. Most of these types of games are not built to complete a 100% but more created to follow the main path and pick what quests/things you find interesting.
At the same time you don't want to miss any of the cool side quests of which some are really good. Maybe a solution would be to rank the quality of the quests in the game. Maybe quests that are 'must play' content should have a different marker compared to 'grind quests' or something.
So you get a better sense of what you can skip or what is there for players who want to max out their levels and unlock everything.
Still it's amazing to see how far video games have come in terms of portraying these epic historic worlds in such incredible detail.
I'm definitely going to pick up Origins as well at some point as it hasn't aged a day for a 3 year old game.
Odyssey definitely made me a fan again.
Can't wait to see what Ray Tracing and a minimum target of 9 teraflops will do for this series
