That's... pretty difficult.
UO had quite a few things going for it.
1) Nowhere was safe(although many places specially remote ones in the wilderness were totally chill). Cities had guards, but you (or someone else) would have to call them. This enabled all sorts of gameplay, including theft. But this was ok because...
2) Items were not too powerful. Classic UO you could get a +9 Sword of Vanquishing or whatever. While it did give you a nice edge (pun intended), it wasn't really worth walking around with it in your normal activities, because of (1) and also...
3) Items stayed in your corpse when you died(until the 'insurance system' that is). So even if you had better gear, you would normally walk around with gear that was easier to replace. And you didn't have to necessarily pay for that because there was...
4) Lots of crafting skills. One could train blacksmithing/mining and mine away. There was a risk involved in these activities, as usual. But other than time, you could have a character that could build any gear you would need - they would be plain boring non-magical, but good enough for any foe in the world, if you knew what you were doing. You would probably do this with another character because...
5) 700 skill cap. So you could grandmaster 7 skills, or spread this across more skills(some characters would get just enough magery to use recall runes). Which also meant that characters would not get exponentially powerful. As long as you had trained the skills you wanted to use, it didn't really matter if your character was one month old or 5 years old. The field was pretty level pretty quick, no one-shotting newbies (really [young] characters were protected) and one could not just spam magic due to...
6) Reagent requirements. Spells were really powerful but really expensive. They took mana and physical reagents that you had to buy in order to cast them. Ran out of reagents? Too bad, time to use that stick. Someone stole them from you? If you survive you are walking home today. If you died, you won't respawn somewhere else, you still need to walk home. But that's fine as you had...
7) Items. Items galore. The land itself had resources. Want some arrows? If you have the skills, go kill some birds and chop trees. Want bandages? Cut up the ressurrection robe (after you found a wandering healer or a friendly player to ressurrect you). Saw a horse? If you have taming, go tame it. I remember dying in the middle of nowhere and returning home fully equipped - bow, arrows, leather armor, horseback. This was revolutionary at that time - and kinda still is in massively multiplayer games. This also leads to memorable moments. Like the time when people got fed up at some 'player killer' and a group stood outside their house. Dude wouldn't come out, so people started bringing tables, chairs, food, and they had a whole picnic in front of his house.
I could go on and on. I am not even touching on the lore, which had many previous games to draw from.
There are games that are harsh(nowhere is safe, dying has stakes, etc), have no classes, player created items, no 'respawns', lots of items to play with, but the exact interplay of gameplay elements is difficult to find. I've considered EVE Online as 'UO in space'. Being a corporate space dystopia it does not have the same "cozy" vibe that even the harshest Ultima Online environments had. Dying was actually fun a lot of the time. Valheim has the closest "vibe" but it's still a little bit on the soft side, and is not an MMO (A MMORPG Valheim, if properly done, would be phenomenal). Rust was mentioned but I do not think it hits the same spot.
Kal Vas Xen Corp!