I'm slightly let down by that Shadow the Hedgehog has been confirmed/shown to be an Assist Trophy, especially after the flame particles when Inceneroar first jumped looked a bit like Shadow's golden spin-aura to me. I was not expecting Shadow the Hedgehog to be playable, but I was "not not" expecting it either.
As for Ken and Inceneroar as fighters, I have no opinion on them other than just "more fighters, more fun".
About Piranha Plant, I'm not sure whether that character will be acquireable for free until a certain time and then paid-for later, or free until a certain time and completely unacquireable afterwards. While I'm definitely getting the game as soon as I can (which includes pre-ordering it from Nintendo's eShop in my case), thus making that irrelevant for me, I still hope that it's the former, as an only-available-before-some-day fighter is not a good idea in my opinion.
Oh, and Spirits (Edit: And/Or the "story" mode)... I'll probably like it more than I'm expecting myself to, but it doesn't seem like something that I'm really interested in right now.