I also think Sidney will be next on the list to die. Regina has threatened Sidney a couple of times. The first was in episode 2, The Thing You Love Most, when she said, “Do you know what I do with things that have no value to me? I throw them away.” Now that Emma has discovered the bug Sidney planted in the vase of flowers, it seems like Sidney is no longer of any value to Regina. Once Regina realizes Kathryn is back, Regina will start to worry she is losing her power to Mr. Gold. Also, in the promo for the rest of the season, there is a scene of Regina smashing a mirror in SB. If it's true that the mirror is still magic in SB, then it seems like she is effectively getting rid of Sidney.
"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy