On the DLC:
Characters needing to come with music and a stage is almost a restriction, as it makes it less likely for characters from IP already in the game getting in as DLC. Thus:
Potential Packs:
SMRPG - Geno, Forest Maze, Music from SMRPG
Banjo-Kazooie - Banjo & Kazooie, Spiral Mountain/Grunty's Lair, Music from B-K & B-T
Rayman - Rayman, Some level, music from Rayman
Minecraft - Steve, Minecraft stage, MC music
Monster Hunter - Male&Female Hunter, MonHun stage, MonHun music
Yokai Watch - Jibanyan, YW Stage, YW music
(this one really hit me as super obvious, he'd be like a rival to Pikachu even).
FE 3 Houses - Edgelord chick with the axe, Some castle from the game, music from the game
Rhythm Heaven - Chorus Kids, RH stage, RH music
Paper Mario - Paper Mario, a stage from PM, Music from PM
That's all I can think of, but those are all the ones that seem realistically possible to me. Reason I'm not mentioning KH or DQ is because it seems more than obvious Geno is the rumored SE character. As for other Capcom IP like DMC, I think Capcom would prioritize MH over DMC, and MH is a better fit for Smash anyway.
In addition to this, if you recall, last game we got a TON of Mii costume as DLC, and with them being Online friendly now, you can be we're gonna get more. They're likely gonna be sold seperate from the indiviual character packs, but released alongside them in sets, like they were last time.
Thus, this actually lines up with Vergeben's claims of there being Minecraft content in the game. Two things, if Nintendo WANTS to make the N64 mini a worthwhile, they're gonna need the Rare game, thus a deal with Microsoft has to be made. And Minecraft as Mii costumes makes SO much sense. You can literally make the Mii Swordfighter be Steve, and even have Creepers be like a mask for them as well, and such. In fact, if we get B-K I can see it coming with a lot of Microsoft owned content as Mii costumes. Minecraft Steve, Creeper mask, Rare characters like Rash (for Brawler), Conker, Joanna Dark (for Gunner), Grunty, Mumbo, Bottles, Tootie, Killer Instinct Characters like Jago and Sabre Wulf, etc... and even Halo content as well. i can easily see Master Chief being a Mii costume for Gunner considering Akira from VF and Heichachi got one.
Like, if you stop to think about this, it makes SOOOOO much sense all of the sudden.