Oh! If we're talking about unpopular opinions, then here's my take:
I think when it comes to when a character is in or not and assist trophies, both sides of the aisle are just too childish and immature. I get it, you didn't get the character you wanted and you have the right to express your disappointment, but just don't take it to such extremes. It's not the end of the world if you don't get your desired character in Smash. But at the same time, the people who insult those disappointed are just as bad. They end up making certain criteria for characters and if your character doesn't fit that mold (say, assist trophy or spirit), then they'll never see their day in Smash. It still baffles me why they like to rub salt in the wound only to be proven wrong in the next Smash Bros when that said character is now a playable character (Ridley and K.Rool being prime examples of this, so they had to latch on to something else). It's just like shooting yourself in the foot. Both sides ruin the fun for the middlemen and to me, it's sad to see such division in certain parts of the Smash community. They're both cancer to the community, at least the extreme sides.
Online mode is a complete dumpster fire. The second worst in the franchise (Brawl being the first). Matchmaking is a hassle and ranking is a joke. Honestly, Smash 4 was way better when it came to online play. At least you could separate the serious players from the casual ones with For Glory and For Fun.
Plant Gang or any Character Gang was obnoxious as hell. No one gives a damn about your precious main. It was a circle jerk if I've ever seen one.
Sakurai has made some questionable choices and he always contradicts himself. In the end, his word is final, not the fans. The fans have some say, but Sakurai is the end all be all for fighter inclusion.
Relevancy... I ain't touching that argument with a nine and a half foot pole. It's subjective so it can take on many meanings.
Anime swordsmen characters aren't as bad as people say they are. For crying out loud, the majority of the roster has some sort of anime aspect to them, so why the complaints? Ok, Fire Emblem is a stretch, but it's not to bad as everyone says.
Subspace Emissary > World of Light. No, you can't change my mind.
Spirit Mode is boring. Fighting for PNGs gets really boring real quick. I kinda miss the trophies.