One mention of Ken and this starts, lol. Alright, we'll put it this way. Ken's ability to adapt is his true ability to come back. IF Ken still has the ability to adapt and so forth, then I'd say he'd be perfect to come back and place top 20 in the nation. I'd say he'd make it into brackets for sure at say Pound V. After that, it's all about how matches come along. Ken does more than play Marth, btw, He DOES play Fox. His understanding of the game is deep, as well. (I am by no means a fan of Ken.)
Also, Amsah is one of the older Smashers in the community and his ability to adapt is amazing. He retired and came back, and he's still mad beast. Is it possible? Yes. Can he be top 3 now? I'm sorry, but no.
Realistically I'd give him top 10 after he shakes the rust off. However, the rate at which the top players in the nation adapt now, is truly amazing. So Ken is dealing with people who are on a level or higher playing field at all times with MORE people than he was before.
I respect the man's talents, and I still say he can realistically be top 10 in the nation, and still make money off of tournaments.
As mentioned before...it's really just speculation. However, anyone with a brain knows that Ken would come back and do some kind of damage if he put the work in.