Diablo 3 does everything Mir does and better but it has no open world PvP and isn't exactly a MMO. I would plead with any Mir server developer to play D3 for a month before releasing their server though. I know a few Mir players that have invested thousands of hours into their D3 characters.
I did search for similar games to Mir a few years ago, found these:
http://co.99.com/
http://eo.99.com/index/
They weren't my cup of tea
Very few open world PvP games thrive in the currently saturated MMO market as they rely too heavily on server population. RF Online, Aika Online and Lineage II were my favourites. ArcheAge could have been the best MMO ever but was ruined by a US company's lack of knowledge of the EU playerbase and their greed in regards to pay to win.
I've never played this game but it does claim to be a "Hardcore, open world PvP focused MMORPG" and it has only recently been re-launched:
http://karosreturns.gamescampus.com/
A lot of Mir2 players went to Cabal when Mir2 EU shut down. I never tried it myself but am liking the look of Cabal 2 which is starting beta late April:
http://cabal2.estgames.com/ (you can sign up for beta now).
If open world PvP is not what you're looking for there is only one game that does dungeons/bosses properly and that's World of Warcraft. WoW PvP has always been piss poor though but its battlegrounds are fun.