I can see him possessing Ramsey or Eva for their powers which I’d assume he’d have in addition to his speed. On the other hand, I can see him possessing someone Barry’s close to like Joe or Iris. Maybe even Nora if she still exists in Earth-Prime’s future.
Personally I’d want to see him take Ramsey because Sendhil Ramamurthy was fantastic but I’d be up for an evil Joe. Come to think of it, we’ve just had mirror Iris so Thawne possessing her is unlikely but it would definitely be in-character. If he does possess someone on Team Flash I think they’ll figure it out pretty quickly so it’s not too similar to mirror Iris.