The Technomancer
card-carrying scientician
Players do want them. WotC know they need to keep sales up as much as possible to keep Hasbro off their backs and a familiar and popular setting, like a popular theme, is a good way to do that.
I'm not particularly concerned about two returns on a row, they have plenty of possibilities for original blocks and it's been obvious since SoM that they were going to aim for a 50/50 split between new and old planes. In order for that to work with things like two originals in a row (Theros --> Tarkir) there needs to also be times with two consecutive returns.
The thing that does seem weird to me is how this fits with the seven year plan. When Rosewater pitched it, it had Innistrad as the first year; this block would be the sixth. INN was supposed to be a risky and experimental block so there's no way it had a planned sequel in the same plan.
I do wonder if the shift to the two block model shuffled around what was on the table. Or else it might have been "revisit somewhere popular" without something locked in yet.