The best game to resurrect as an MMO would be URU: COMPLETE CHRONICLES
http://www.gog.com/game/uru_complete_chronicles for starters it was an MMO, and it is already on GOG, so those two things count in it's favour.
When another company used to stream/rent out versions of URU, they provided the MMO aspect. But they had a monthly membership fee for the service, so if it was popular and keep people subscribed that would have generated profit for them.
So only the cost of running it, if the publisher would hand over the server application and sign off on letting it be done, are issues.