I'm not salty about any of these, but I was disappointed nonetheless.
Spring Man. I loved Arms, even though it was lacking in enough content to keep me interested for a long time. Still, I thought Spring Man being in Smash about a year and a half after Arms came out would revive the community, maybe get me back in the game, and be an interesting fighter. I remember the fake leak with Spring Man and Decidueye and I knew it was fake, but I still really wanted him in the game. I'm glad Arms at least got some representation when the game launched, with Spring Man being an assist trophy, but it just wasn't in the way I hoped for. I'm glad Arms is getting a fighter, but it's gonna be weird without it being the mascot of the game.
Shovel Knight. Shovel Knight's amiibo made me think he was at least gonna get something in smash, and while he did get an assist trophy, I just wish he was playable. It's still pretty crazy that an indie game got some representation besides a spirit or a random song, but it just wasn't what I hoped for.