86: Eighty Six is highly praised, so hopefully it'll be good. I haven't read it, but I believe shinta does.
https://anichart.net/Spring-2021
I've Been Killing Slimes for 300 Years and Maxed Out My Level - should be a nice slice of life isekai if you like that kind of thing.
Eden's Zero - newest series from Hiro Mashima (Fairy Tail, Rave Master)
Shakunetsu Kabaddi - sports, so will give it a try.
How NOT to Summon a Demon Lord - never finished the first series. Didn't think much of it at the time, so probably won't get back to it.
Zombie Land Saga - didn't watch the first series, so won't be watching the sequel, is it any good?
Shaman King (2021) - will watch for sure, even if it's just for nostalgic reasons. I believe it'll be more faithful to the manga, since I think halfway through in the original it deviated. Many of the same VAs are returning, hopefully the new ones don't mess it up.
NOMAD: Megalo Box 2 - liked the first one.
Combatants Will Be Dispatched! - from Akatsuki Natsume (Konosuba/Hataage!)
Other than those observations, will need to check the rest to see what are they about to decide. Of course, will watch all the usual sequels like
My Hero Academia and whichever other is returning.
Edit: Some more that will deserve a look from me...
This ones look to be the better ones:
To Your Eternity: Fumetsu no Anata e
Jouran: THE PRINCESS OF SNOW AND BLOOD
MARS RED
Shadows House
Vivy: Fluorite Eye’s Song
Full Dive: This Ultimate Next-Gen Full Dive RPG Is Even Shittier than Real Life!
These others I may give a look anyways:
Tokyo Revengers
The Saint's Magic Power is Omnipotent
Dragon Goes House-Hunting
Cestvs: The Roman Fighter