This may qualify as an unpopular opinion, but Arlo (yes, the Nintendo puppet YouTuber that looks like a mix of Grover and Cookie Monster) stated that the inclusion of Piranha Plant means that ANYONE can literally make it in Smash and that you don't need to be a super mega requested hype character to be part of the roster.
I have very mixed feelings about that train of thought. On one hand, I understand where he is coming from as Piranha Plant does open the floodgates to many characters coming in and that there is no need for every fighter to be super mega hype characters. It allows the team to experiment with character choices and may create a very unique, varied and fun roster in the process...
ON THE OTHER HAND...
When Arlo said that, he seemed to have forgotten that even if Ultimate has an incredible roster, there are still many characters that are highly requested and were either forgotten or relegated to Assist Trophies. If Piranha Plant can be made into a fighter so can Isaac. And yes, I know I am beating on a dead horse here, but the problem with Assist Trophies I feel is that they are not balanced. Like you have some characters that ARE built for being assist characters (like Ouendan/Elite Beat Agents, Chibi Robo) or to be a nuisance like enemy characters (like Kremlings etc.). Yet ATs include characters like Isaac, Shadow, Spring Man and, yes, even Waluigi, that CAN be turned into full fledged fighters with enough ingenuity. So the whole "the character doesn't have enough to be a fighter" is dead when they made Pirahna Plant, a character that screams Assist Trophy/Item into a fighter.