If it doesn't have to be relatively new, and you don't have a problem with the somewhat grey area of server emulation, then you can pick almost any old MMO that you liked, dead or otherwise, do some digging, and turn up a handful of player/community-run servers and clients.
WoW is/was the most prolific of these, (last I checked anyway, but that's been awhile,) and Ultima Online before them. Both still exist.
Quality of these servers can be iffy, but most that have been around more than a year will have refined their server and likely have patched many of the bugs that were present in whatever version of the server they started with. As long as you have some technical skill, and are willing to accept that there will be bugs, then it shouldn't be an issue.
There are also numerous gameplay modifications that are typically available on these servers, as well as plain vanilla gameplay. For example, some I've played have had 10x exp/gold multipliers to reduce grind or give you a huge starter pack to get you to the late-game content, skipping grind altogether.
"[insert game name] private server" in your chosen search engine, and I would be surprised if you didn't get a few pages of relevant results.
Just an option to consider if there were ever any MMOs you liked, especially if there were any you both liked.