In many online RPGs you get daily and Clan related quests. Season quests are fun, except set dungeon, but they are cleared in the first week maybe in the second. Also the rewards can vary from items, to mats even to a limited time to convert mats to death breaths. There is a time where you have done so many Grifts you have mats from 5 to 6.000 and you can't use them because you don't have death breaths so you do Rifts upon Rifts waisting time to gather them.
I totally agree. I like that my four closest gaming friends are part of my clan, but honestly -- what's the point of clans? What additional benefit has Blizzard set up for clans? No clan stash. No way to trade with clan members. No clan perks -- achieving clan experience to raise clan level in order to purchase perks.
A few years ago blizz didn't even complete a PvP aspect with proper maping, being competition a core aspcect of the D3 i dont' expect to have clan related features even if that helps to make a much better game. Since seassons the game was focused on nerfing, buffing and creating new builds and thats it (you can say they did add some other content but to what is really relevant not so much).
I think the challenge with clan based quests is making them interesting enough to hold people's interest.
Take Ghost Recon Wildlands for example. They implemented seasons with weekly objectives that boiled down to 3x solo objectives, a clan objective and a community objective. This usually boiled down to kill X number of enemies in Y manner in Z location. I don't see Diablo 3 being much different if they're going to go for things that any player can help contribute towards. Make it too hard and it'll alienate players, make it too easy and it won't hold much interest in the long run.
The rewards are also up for debate... if it's mats, then the strongest clans will get a leg up in seasons. You'll also have some players lose interest quickly leaving it to a handful of players to continue with the quest tasks and those players will eventually stop as they'll be sick of doing the work for everyone else.