Don't worry, anne already pointed out that they don't necessarily have to leave the group to "go solo". For example, Taeyeon's "Can You Hear Me..." was a solo project, but she's still very much a part of SNSD.
Anyway, I think Nicole has the strongest chance of being the first to get a solo project because she's so well-rounded. Decent vocally, bilingual, and able to rap (sort of xP).