He's played majority of his early pro career as a right wing while playing center in the junior leagues. Think he stated earlier that he felt most comfortable in the middle on an interview once. As others said his position in the future propably depends on the team that drafts him. He has all the tools to play center for having high hockey IQ, visiong and passing ability. While on the other hand possesing the skills, speed and the physical side to play as a powerforward.
If Rantanen remains in Finland after the season and has the chance to play in the World juniors I'd definately prefer him as a center. Why they didn't even try him there the last time around is beyond me. The team literally had none high caliber centers and you could have paired him up with Puljujärvi f.ex. While looking further ahead, it'd propably suits the national team better to play Rantanen on the right wing in the long run. The ability to play both roles should raise the stocks for the upcoming draft.
There's been a lot of suprise fallers in the recent draft years (including Forsberg, Tarasenko, Kapanen and so forth) but can't see this happening with Rantanen. He really doesn't have much if any question marks other than not being American player.