Many of the other actors attached to the production have been quite busy with their own schedules lately, so that doesn’t leave a whole lot of options, unless the role is actually a small cameo-like appearance and not a major reprisal of the character. That would leave a lot more options for who the character might be exactly.
Both are equally possible, although after building up all this hype about the return of the mysterious character, the producers could disappoint their fans if they do it in such a short, quick fashion. We suspect that they are probably planning something a bit more major and a more thoroughly developed featuring of the character over at least several episodes.
We’ll have to wait until season 3 arrives to see where things will go, but this will probably take a while. The show isn’t expected to resume until 2017, which adds a pretty long pause between its second and third season.
But the situation was somewhat similar with the transition between the previous two seasons anyway, as fans had to wait over a year to get their new episodes. After everyone was convinced that the producers of the show are putting all that time to good use, people seem mostly content with the idea of waiting that long for another season, and this would of course also give fans a great opportunity to discuss all that could happen.