It wouldn't be a MMO if it were offline and it wouldn't make any profits if it were a standalone RPG, all you will find will be either flash games or extremely bad or unfinished random indie games, your best bet are total conversion mods for certain games (there are few that manage to form a campaign tho).
Lol sounds like what you're looking for is a regular RPG, not an MMO. Can't help ya out with the free part though because I'm not well versed in free RPGs.
The online games that are even close to a OFFLINE MMORPG that i can think of, are games from the .hack series, which are neither free nor a actually MMORPGs, the also arent on PC. There are plenty of rpgs games out there with multi-player though, but most likey not free, seeing how even f2p games are not really free
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