I haven't played Arx Fatalis or Underrail yet (although I want to), but your post reminded me to also suggest the Ultima Underworld games, which I believe inspired Arx Fatalis. And actually I haven't played them yet either, but they're on my list.

It was originally intented to be Ultima Underworld III but developers were unable to get permission from EA for the title so they made it inspired game instead. But if you add Ultima Underworlds you pretty much have to add all other dungeon crawler RPG's from Rogue(likes) to Dungeon Master, Eye of Beholder and Legend of Grimrock and anything between. :-p

I've heard that the Ultima Underworld games set themselves apart from other dungeon crawler RPGs by having a more fleshed out and believable subterranean world, which is why I suggested them. But again, I haven't played them myself. Isn't the premise that the entire world is subterranean? This would contrast with most dungeon crawler RPGs, which have standard fantasy worlds but just happen to take place in dungeons.

I realize it's not isometric, but I'd definitely recommend Fallout: New Vegas. It has similar quest design to Arcanum (multiple approach, multiple solutions). It doesn't feature black and white factions, even though it might appear so at first, it has an interesting reputation system determining what quests are available to you. Setting is very interesting '50s Americana vision of the future, like in other Fallout games) and there are some very interesting locations to visit. Sadly, it is not steampunk.

Depending on how old you want to go, you could also take a look at the SSI Dark Sun games as well as the Buck Rodgers Gold Box games.

I love me some Dark Sun.

I'll toss in a game I picked up a little while ago here on GOG.com: Wizardry 8. It's a real classic with a very old-school feel (but quite easy to play). It's the first Wizardry game I've played since Proving Ground of the Mad Overlord and Knight of Diamonds in the early 1980s, and I've been really impressed.

It's fantasy because it has elves, dwarves, hobbits, and magic, but it also has a bunch of alien races like T'Rang, Umpani, Mook, and Rapax, as well as spaceships, space travel, guns, robots, and all sorts of other sci-fi items.

I second this recommendation. Wizardry 8 is a great RPG, with an amazing and unusual setting. Just be warned, this game is more about tactical fights and exploration than about role playing. Think Might and Magic titles. It also has one of the most interesting RPG mechanics.

I sooooo wanted to play these back in the day when they came out and was never able to get them (I was 12), and have wanted to play them to this day. I wish they'd come to GOG.

Also, I just started playing Inquisitor, and it is quite good so far.

I agree with an above poster in Dark Sun. All the old DOS games take about about 30 min to an hour to get accustomed to, but once you figure out what keys and buttons do what you're home free. Dark Sun is yet another D&D game but it is essentially set in a Mad Max-like wasteland and is the only game I know of using that license that features the Psionic class option. There is a sequel as well.
Inquisitor is quite good, and has gotten a bit of a bad rap for it's combat above all else. And it is pretty much junk. But I'd rather play Inquisitor-style combat for an eternity than deal with Arcanum's engine.

I know someone above me already mentioned Planescape: Tornment, but I think it bears repeating.
Why? Because to me it's one of the most interesting settings and stories ever told in a computer RPG.

Arcanum's engine grew on me over time and I actually enjoyed the combat quite a bit in that game once I got used to the feel. Though, the only way it was enjoyable was fast-paced turn-based, so if you haven't tried it that way, do!

For those who happen to have Neverwinter Nights 1, I'd recommend "Hex Coda" or "Elegia Eternum".
Inofficial professional quality modules that have unusual settings and strong as hell storylines.

I've found 'A Dance with Rogues' to be an excellent module for NWN. Not much fighting, lots of role playing. Must finish part two one of these days.

There's a huge pile of absolutely fantastic NWN modules, but those two are a perfect fit for "something different".
Not that "A Dance with Rogues" is standard Tolkien stuff by any means. Opinions differ, I like.

But that leads to two more things.
If it's the rogue roleplaying thieverying thing that appeals, Honor among Thieves is great.
If it's the roleplaying combined with XXX, I'd suggest "The Bastard of Kosigan" series (as in illegitimate son).

I liked it most because there wasn't any grind and each fight was something special. Playing as a rogue/thief always appealed to me in D&D and this module was a perfect fit. XXX stuff wasn't something I was looking for per se, but it fits very nicely into the general tone of this particular module. Seedy underworld where there is no such thing as a kind a good rogue.

I'll definitely give those two a try once I reinstall NWN. Most of the modules I've played were from your recommendations from NWN subforum here.