He has several options, Astana ain't one of them to be honest.
If he has enough confidence in himself to be able to get Giro and / or Vuelta leadership at Sky he should sign for a big paycheck.
Tinkoff would be another option. If Sagan leaves there's space in salaris and as Contador is likely to target one last Tour in 2016 the door is wide open for Giro + Tour leadership.
Lampre has had a little renessaince this season and they may be looking for a Giro leader and give him decent support with their climbing talents. That said, Lampre ain't gonna give anyone a competitive paycheck to Sky / Tinkoff, unless Merida takes over as the main-sponsor in 2016. So they'll probably try to sign someone like Kruijswijk, Hirt, etc.
Movistar has Quintana and Valverde around, like Astana has Aru and Nibali. Why should he do a transfer if he stays the teams number 3 in 2016 as long as he can be at least number 2 elsewhere. He still could lead them at the Giro and be co-captain at the Vuelta though.
If he leaves,I wouldn't be suprised if he rides the Vuelta for his new team already. There's an easy deal to be made where nearly everyone takes profit, except Aru.