I chose Dwarves and Lothlorien, because I'd really like to play them ^^ (especially Dwarves (though no thanks to the Hobbit movies : P), since I already liked what you did with them in 3.8 : ).
Realisticaly though, if we're talking about releasing 2 factions at the same time, it wouldn't seem proper to have 2 good factions released, so I would rather go for : Dwarves | Misty-Mountains, because Dwarves are cool (and their new city design looks awsome) and Misty-Mountains seems like the perfect enemy for them (and I'm looking forward to the many changes they got in 4.0).