Some good or at least decent stealth action games, some are open world, some not:

• Far Cry: most of these can be played in that way
• The modern Tomb Raider games (since 2013 or so)
• The Sniper Elite series
• Tom Clancy's Ghost Recon series
• The Hitman series
• Dishonored + Dishonored 2 + all addons/DLCs
• Deus Ex Human Revolution & Mankind Divided (also the very first Deus Ex if you can tolerate its "graphical fidelity" nowadays)
• Deathloop, but it's worse than Dishonored and much less "serious" than the other games on this list

Red Dead Redemption (both) are basically cowboy AC but instead of assassins/Templars, it's just rival gangs in the old west. Even has 'tracking mode/eagle vision,' 'Deadeye/VATS,' and all that jazz.

The Batman Arkham games are good, though I've heard mixed things about Arkham Knight, very stealthy/assassin-y games with Detective/Eagle Vision and sneak takedowns.

If you're into more exploration based fantasy rpgs with good combat, Dragons Dogma/Dark Arisen is really good. Pick from warrior/mage/rogue/mix of any 2, and your combat changes based on that and your weapons/magics. Throw smaller enemies like goblins at each other, grab onto a griffin or cyclops to take out its wings or eye. First one is great but heard mixed things about the second, and mtx.

Ghost of Tsushima is basically just honorary Assassin's Creed, so you could play that.

Rise of the Ronin is also underrated, it's basically a Ghost of Tsushima with better combat but worse everything else. But still has you covered on the open world slop front.

Both these games have tons of checklist stuff on the map to do and camps to clear and so on.

Lots of them. Everything Ubisoft has done for the last 10 years is the same game with different skins.

Star Wars Fallen Order and the sequel Survivor. Even if you don’t like star wars you’ll enjoy the game and gameplay if you enjoy assassins creed.

Edit:

Also the Mad Max game from 2015

I don't know why I like the Assassin's Creed games so much. They've been getting progressively worse for like 10 plus years now but I still like them. I even liked the Viking one, well I tried to. Haven't tried the newest couple of ones though. Mirage doesn't look very appealing maybe when it gets really cheap.

Certainly nothing really scratches the same itch exactly. There are a few games that almost do though that might appeal to you. The Shadow of Mordor games were kind of similar, not quite but in the ballpark. Even if they do offend me horribly as a fan of the lore. Far Cry 3 also came close though in a wildly different manner. Thats the only Far Cry game I've played but maybe thay would be a good place to look.

Far Cry 4

I was told Breath of the Wild and Crimson Desert have a similar vibe, but I'm not sure as I've not played them.

every assassins creed game is basically identical, so pick any from that series

A lot of people are going to probably suggest Stardew Vally to you, as it’s the 500lb gorilla of cozy games, but for me hands down my favourite cozy game is Spiritfarer. It’s got quite a bit of exploration and bit resource management, but the real draw is the story line I’ll try not to spoil it but it takes place in a sort of limbo afterlife and your job is to help your friends move on to the next plane.