Say I had a long lost uncle with no children who left billions to me when he died and I used those billions to buy Activision-Blizzard. How difficult would it be to convert World Of Warcraft from a MMORPG into a DRM-free single-player and multi-player experience using all the preexisting assets?

Redesign the whole gameplay from the ground up and make it single player and no online features whatsoever.

And add better and unique quest instead of just "Kill/Get X amount of things"

No daily quest.

Expand the combat to make it more like Dragon Age Origins and give the option to control party members.

Raids don't "lock" until every tuesday.

More story driven presentation, how the expansions work well. The base game is Vanilla and Burning Crusade and Wrath to a degree. The rest of Wrath of the Lich King can be treated as another campaign like Neverwinter Nights expansion.

The same can be said for the later expansions.

And make the graphics all up to the current WOD/Legion standard.

Moderately difficult.
There are already customized WoW servers available for download that let you play the game with bots. Granted, these bots are not necessarily of the highest quality, but if a professional team would work on developing and implementing them, it could pretty much work.

Why are you asking? We all realize it won't happen anyway. :D

Sorry, uncle Barry. Just count the billions they have made out with it, in its current state and model. Then, pit all those billions they earned and compare, against the billions you got from your late uncle. The maths don't fit. The bill cannot be met. There is no profit for those people in changing model. Sorry.

I misread say as so. was a bit of a trip for a second. I thought you were making a serious inquiry.

Don't try converting the existing game.

Make another game in the Warcraft universe.

Say 'fuck WoW' and make Warcraft 4.

But the world of Azeroth is so much more intersting as in Open World RPG :(

How difficult would be to change football (US or European, your call) to a single player game? Similar difficulty.

WoW (and most other MMOs) are based on players interacting with each other. While it is possible (or was, haven't played WoW for more than 6 years) to play quite a bit solo, you do miss a hell of a lot of the story if you can't group with other players.

So, if you did have said billions, it would be best to start from scratch. Especially since WoW has at least 2 lore lines you'd need to give the player access to (Horde and Alliance). Add to it class and race based lore, as well as profession based one, and you do have a mess on your hand.

So no, while you can (in theory) change WoW to a single player game, the cost of it would not be worth it.

Before you do all that, you should first make it fun.

To enjoy an MMO, first find a group of people to play with. That's the basis of enjoying any MMO.

Can a MMO be DRM-free?

Define DRM-Free.

The solution is to publicly release the server so that people can set up their own servers. Then it could be DRM-free.

WoW accounts require a serial. And from what I recall, they do depend on the server you attempt to connect. Don't recall if a US account can be used on a European server, or if you can use a serial to create either kind of account.
So other than the servers, you'd also need to modify the account system.