Last Epoch and Grim Dawn are probably most in line with Diablo, I think.

People have mentioned Path Of Exile, and I've played a lot of it, but I don't think it feels particularly like Diablo any more, even though it started out that way. It's quite unforgiving, and even a lot of experienced players feel like they need to follow build guides rather than work things out for themselves. Its learning curve is hundreds or thousands of hours long. Of course, the reason for that is that it has incredible depth, variety and complexity, which may be a selling point or a deterrent depending on what you like! I definitely like the complexity of it myself, but it's very overwhelming when you're new. The reason I don't think it's all that in line with Diablo these days, though, is simply the pacing of the gameplay. You blow up screens of enemies at a time, and your deaths are often so fast that you're not really sure what killed you.

Path Of Exile also heavily revolves around its trading economy. Item drop rates are balanced around players being able to trade for them, which makes trading somewhat mandatory (unless you're a bit of a masochist). The economy is fairly complex, with there being a lot of different currencies, and quite a lot of factors that can affect the value of an item. I'll let you decide whether you find this appealing or not - some people do, some people don't! I do think it causes some issues with the balance and progression of the game, but it's interesting to say the least, even if you wish you didn't have to engage with it.

Grim Dawn feels a little mechanically dated at this point but it's still solid. It's got some good builds, the dual-class system and constellations system make for some interesting variety. It's got an offline mode, as well as online co-op play. Its real selling point, though, at least for me, is it's absolutely soaked with atmosphere. It's very, well, grim, but the world is really immersive and it has a great setting in general with a solid story and some great lore. It also has quite a lot of mods available (including the Reign Of Terror mod I mentioned in another comment in the thread that adds the entire Diablo 2 campaign and all its classes to Grim Dawn).

Last Epoch is more mechanically interesting than Grim Dawn, I think, but it's lacking in the story and world-building. It's still in early access, although its full release is next week. It has quite a lot of depth and complexity, but it's all done in an intuitive way that means you can jump into the game blindly and work things out for yourself fairly easily. It has a good variety of skills, and the fact that each skill has its own fairly comprehensive skill tree means you can play the same skills in very different ways. It has a wonderful itemisation system that does a great job of making you actually engage with the loot you find on the floor (which is an issue in other loot games), and some of the best crafting I've ever seen in a game. The dev team also manages to come up with some really creative and somewhat intuitive solutions to things they perceive as issues in other ARPGs.

Last Epoch's biggest drawback is that its endgame is currently a little lacking in comparison to POE (which has a very rich and deep endgame, but is also a ten-year-old game that's been updated constantly). It's still far, far better than Diablo 4's, though, and will obviously only improve as more is added. Last Epoch has some truly brilliant systems in place for the devs to build off - and frankly, I still think it's great now - but it'll only get better as more content gets added over time.

I love all three games I've talked about for different reasons, and honestly, they're all well worth playing!