LOVE THIS SHOW!
I have 3 theories for the mystery man.
The first one being the one I think is more likely.
1.)
Mystery man is either the March Hare or White Rabbit. There is the obvious "Hatter" sign, a bunch of clocks, monocles hanging on the wall by the door and if you look at his desk it has what looks to be keyholes as a design. I could be just crazy but it made sense to me!
2.) Mystery man is Rumple's son and Regina has known where he is and is the one who sent the postcard. It's in Regina's best interest that Rumple's son return in hopes of stopping him from doing magic and therefor making her the most powerful.
3.) This one is my mom's. She thinks mystery man could be captain hook. Now the mystery man has 2 hands so I'm not sure how possible it is. But it would explain all the clocks in his apartment!