It's funny - before Sm4sh DLC, I was pretty anti third party. Now...half of my most wanted are.
#1 - Dovahkiin (Elder Scrolls series, Bethesda)
I want the Dragonborn so bad it's not even funny. Skyrim may be getting older at this point, but it was used to show off the capabilities of the Switch in its initial reveal trailer (as the second game shown off after BotW, I might add. Skyrim was announced for Switch before Mario was shown off...). Skyrim also managed a 40/40 in Famitsu, which isn't something every Western game can claim. With the fact that BotW definitely shows some inspiration from Skyrim, and that Skyrim's Switch port did put in some BotW content as well, it'd be great so see Link and the Dragonborn cross blades. The Dragonborn does have a relatively consistent look in all of the marketing but could use alternate race/gender options as alt costumes, and he could use several options for his normals (marketing images have shown him dual-wielding or using a shield, and archery is also a possibility) and Dragon Shouts as his specials, which would make for a unique moveset.
#2 - Tails (Sonic series, SEGA)
Miles Prower is the most deserving character for Smash in my eyes. With Sonic now a three-time veteran and his numerous crossovers with the Mario franchise, it would make sense that Sonic gets a second character. The best option for me would be Sonic's original player two, who is still considered his BFF. Sonic's moveset also borrows a lot from his earlier games, which would make sense if Tails were to be an easy echo or semi-clone. Shadow may be more popular in the Sonic fandom, but Tails is very recognizable to those who aren't as close to the series. Also, with Shadow having been an assist and Knuckles' new assist trophy, it feels wrong for the original friend to be left out completely.
#3 - Ivy (Soul Calibur series, Namco)
Soul Calibur isn't Namco's most popular or best performing fighting game series (but hopefully SC6 can turn things around), but it is still very well known. Ivy's known mostly for her unique whip sword, complex movesets, and one or two other things. While her default costumes may be a bit skimpy (understatement of the year), she has taken more conservative alternates before, and Bayonetta has shown that even she can be toned down. Still, the idea of a fighter using a whip sword would make a very interesting close and far range moveset, and the battles between her and Bayo would be legendary.
#4 - Nightmare (Soul Calibur series, Namco)
Let's face it - if Soul Calibur gets a character, it'll be the one in the series logo. Still, Nightmare's got a lot going for him as well considering he acts as the host for the evil sword. Plus, he would get his alter ego Siegfried as an echo or alt costume. After the crossover in SCII where Link joined the Soul Calibur cast, it would be fun to see them be able to cross back one more time.
...honestly, that's all I really have. But #1 and #2 are leagues above anything else...