Sheik could well be the best character in the game when you consider her MU's, bonkers neutral and recovery, and plethora of options she has at her disposal.
But do I see Sheik getting nerfed in the potential Switch version? Actually, no.
Yeah, at the highest of top level play, Sheik is absolutely amazing, but she does require so much precision and tech skill that at overall lower level, Sheik's don't prevail as much as overall easier to use characters like Mario, Sonic, Cloud and even Captain Falcon. Sheik requires a whole lot more consistency than the other characters (especially important with her being so light now).
As a disclaimer, this is in no way indicative to other areas as a whole, but based on my own experience, my local Smash 4 weekly has multiple players using top tiers. There are Diddy's, Mario's, Cloud's (by the f***ing shovel full), Sonic's, Fox's, Mewtwo's etc, and there is one guy who uses Sheik... he doesn't place as highly as the other players using the other top tiers. Interestingly enough, the other characters are a lot easier to use overall...
But yeah, I still think Sheik is potentially the best character in the game, even if it is just from personal experience. I'm no top player, so this is just my experiences, but I've been backed into a wall on far more occasions by legitimately good Sheik players than I have legitimately good Diddy's, Bayonetta's and Mario's for example. A truly optimised Sheik is just a lot more troublesome to challenge than literally any other character (you could argue Diddy holding a banana is worse I suppose).