The AC series is weird.
The side content in every game post AC2 is both 80% of the game and is also almost entirely collect-a-thons, mini-games, side-systems and sidequests. Yet in the good ACs, the OCD stuff is fun for a while and makes it seem like there's a lot of different things you can be doing at any time. Yet still, there's no arguing that at some point in AC:B -> ACIV, you're going "goddamnit, why am I wasting 5 mins of my life to run/sail 300 feet to grab a fragment to see a 35/62 number pop up and eventually a 62/62 number and then maybe get an achievement after a dozen wasted hours"
Honestly I think in the perfect AC game, the solution to have lots of OCD side content but also keep it from getting tedious collect-a-thon is to keep all the different things to collect, but just not make many of them. Instead of 60-90 fragments and chests at sea on little islands with nothing else on them that are all copy & pasted, how about 10-20? I think if they toned down the collect-a-thon stuff and kept the mission stuff like Assassination Contracts, Race missions (which kind of sad they don't do these anymore; I think AC:B was the last time?), and other sidequests then it's be a completely fun game with lots of content that never feels like a bore.
Btw, the worst stuff in this game (which is pretty minor, since overall it's quite good) is when they force what should be optional objectives into the main story missions. Like "do this 15 min sandbox section, but you can't be detected once or you instantly lose". Like I said earlier, AC at its best is when it's a real sandbox game and you can play perfect stealth if you want but that's because it's your own choice and not the game's choice to play extremely linear. There's also ship tailing missions that are like the eavesdropping missions on foot but even worse with guard ships that have detection scopes. Those things suck, but thankfully at least you can rate all the main missions in the game, so the shitty ones get 1 star and the good ones get 5 star. I think Ubisoft will actually take fan feedback into consideration since they want the games to sell as much as possible. So if everyone rates the eavesdropping missions 1 star, I can see them telling the designers to cut back on them or remove them entirely 1 or 2 games down the road.
