Anna always said she wanted to come back as Rogue, but only if she's able to do some action scenes and Bryan always had good plans for Rogue.
Nowadays TB ends filming next month, XMDOFP won't end until October and she officialy doesn't have another viable project for this year (Black Wings has my Angel is in limbo), so there're enough time to acomodate her recent maternity with the filming schedule.
Personaly I don't think her role is going to be small, she's my favorite for the timetravel and seeing that she stopped filming when mostof the OT actors were still in Montreal makes me belive it. For me that means Rogue was in the past while the OT team was fighting in the future. She wasn't show with the OT actors chairs, and I think it was because her chair will belong to the FC team. All that, plus Anna comments about an active role, being one of the first cast anounced, all the free time she's going to have after June and her increased popularity thanks to True Blood makes perfect sense for me. Just saying...