A MM sequel would be amazing. OoT/MM style gameplay/characters with revamped graphics would be very cool, not to mention flashbacks to stuff that happened in OoT/MM. The Hero of Time Link is by far the most developed Link character and is overall the most interesting. The rumor that Link will not use the Master Sword in this new game supports the theory that you will play as the Hero of Time after MM (since Link puts the MS back in the pedestal and leaves)
I'd really like to see some completely original dungeon themes (not just basic elements like grass, fire, water). Going to the Sacred Realm and possible doing the Light Temple would be AMAZING.
There is a large gap between the Hero of Time story and WW/TP. I think this would be the perfect time to fill that gap.
My thoughts after seeing the artwork:
The MS probably "transforms" into this girl since Link is shown swordless in the artwork while she is in non-MS form. It's also possible that you don't use her as the MS in the game, she may just be it's "spirit". There's also a good chance that this girl and the Great Fairies from WW are somehow related since they look so much alike, especially the little girl fairy that gives you the fire/ice arrows). If I had to guess, I'd say that the girl in the artwork is the "Great Fairy of the Master Sword". None of this disproves that it could be a MM sequel.
Here's a comparison picture I made (colored circles correspond to the other circle of the same color):
Notice how Link is swordless and seems to be protecting the MS girl.
Here are two other excellent comparison someone made:
