While we wouldn’t necessarily be surprised to see villain-centric movies centering on a character like Venom, or even an ensemble movie for The Thunderbolts within the next few years (especially in the wake of Suicide Squad), the idea of Nebula taking center stage for a solo movie of her very own just doesn’t seem to be in the cards right now.
Now for the part where I rain on your parade: as much as we agree with James Gunn, and as great as a Nebula solo movie sounds, it’s not very likely at this point.
Marvel has not made a single movie centering on a villainous character to date, and the prospect of them starting with someone as obscure as Nebula seems fairly slim.