Okay, I'm not going to comment on the idea, but I'll address the technical side of things, since I have experience there.
In the core of it, you have a 3

RPG, open world, sandbox, massively multiplayer.
On the programming side of things, things are better than ever with many AAA engines available practically for free. However, a game is so much more than an engine. Without MMO level of multiplayer that's probably $500k just in base coding. Add another $250k for MMO component, plus support costs go up astronomically.
Art: If you want original art of a AAA level (not too fancy, let's say Skyrim), that will be around 3-6 million, if invested wisely (i.e. modular models). Again luckily, you needn't pay nearly as much if you buy premade art, but that means that 1) you have very little control of details and 2) other games may have the exact same models in them.
But if you are doing this smartly, and don't have any outlandish requests (e.g. My races all have four legs and twelve arms!) you can probably get everything for $200k.
Story... The good thing about MMOs is you don't need many writers.. Sure, there is the overall arching story, but that takes one person to write.. Writing NPC dialogs and such takes a lot, LOT longer. Consider it an average of $200 per NPC of reasonable depth (i.e. not just "show me your wares [open store]").
Voice acting.. If you want your NPCs fully voice acted add another $500 to the per NPC cost. If you don't do full voiceovers but just voice sets (i.e. only generic "hello","bye" and similar lines per set), it goes down a lot. You can probably get a good voice set for $200.
Music - You'll probably want background music, a theme, etc.. You can either pay a composer or licence an existing score. Either way, set aside say $10k for it, you'll need it.
That's the very rough outline of what you need to make the game. Now, if you want to sell it to actually make a profit, you have two options:
1) Hope it goes viral in some way. The payout here is pretty good, but the chances are somewhere around 0.005%, based on existing data. If it is AAA quality, then probably something like 0.3%
2) Get a AAA marketing budget. This starts at around a million dollars and doesn't really have an upper bound. The more you put in, the more you get out of it.