I'm going to begin by testing out my mains from previous games, them being Ness, Falco, and Olimar, though with Ollie I'm probably going to use an Alph skin, maybe the one that colors him like Brittany.
As for Veterans that I haven't mained, King Dedede's changes look phenomenal. I can't wait to try out his newly buffed self.
Newcomer-wise, I have a few I want to try. I'm definitely a roleplaying kind of guy, so I'll be doing a lot of things with my Mii Fighter regardless. I'm thinking Gunner looks the most fun of the three Mii Fighter styles, followed by Sword, then Brawler. Villager doesn't look hugely viable, based on the demo, but he does look like a ton of fun to play as, so I'll be giving him a fair crack as well. Bowser Jr. is actually one of my favorite characters in the Mario universe, and that Shadow Mario Final Smash that he's got going for him looks incredible. I may play as Ludwig instead of Bowser Jr., though, just to shake things up a bit.
Of course, the one I'm going to work the hardest on is Duck Hunt. That Banjo-Kazooie alternate skin is to die for, and I will do everything I can to make the duo my mains for the skin alone. I won't force it, though. If I'm just straight-up better with someone else, I'll use the someone else instead. I'm pretty confident that I can master the Dog and Duck, though. Confident, and very hopeful.