If endgame is a requirement, then Last Epoch, Grim Dawn, and PoE are likely your best bet. There is also Diablo 4 obviously, but that pretty much is D3.
Last Epoch is supposedly bucking the trend, by having new content come both to their Season Realms and Non-Season Realms.
Grim Dawn dont have seasons in the first place.
PoE is as season-infected as D3.
In a “kinda Diablo-like, but also feels different” category, there are Nioh 1 and 2, with lots of endgame content, similar to Diablo. But with a more Souls like combat system.
Tbh, Diablo fans are sleeping on those games.
Now, if it doesnt have to be super long endgame like in A-RPGs, then the options increase significantly. Like, lots of JRPGs around with substantial, but definitely not endless, endgame content. Many of the Final Fantasy games, Xenoblade Chronicles, Dragon Quest and what not. On the flipside, they tend to have fairly low replayability after that endgame. Starting a new character will be the same experience again.
And of course, if endless content is important, there are all the various MMORPGs (WoW, Final Fantasy 14, ESO, Lost Ark (more Diablo like than the others) etc.
Personally though, I would rate a good base game, rather than just a good endgame, higher.
Like, Baldurs Gate 3 might not technically have any endgame. Instead it has enough game in it, for many playthroughs.
Or Elden Ring. It does have some endgame, but the focus is on the base game. Which is just good enough to carry its own, over and over, without needing to mostly “artificially” split the large amount of content into separate categories.