Based off of Nintendo's statement at their investors meeting here's how I could see DLC going:
- Piranha Plant - meant for base game, was low priority and didn't get finished on time
- Joker - meant to drum up hype in the West and represent Nintendo's relationship with Atlus. Likely opened the door for Persona 5 Switch (when it's revealed)
- Erdrick - meant to drum up hype in the East and represent Nintendo's relationship with (Square) Enix (remember that Square and Enix used to be separate). Secondary reasons are to push sales in the East and advertise the series in the West.
- Banjo - meant to appeal to the core Smash Bros. fanbase alongside older fans and improves relations between Nintendo and Microsoft
- Leon Kennedy - meant to appeal to a more hardcore demographic and capitalize on the recent surge in popularity for Resident Evil to drum up hype and sales.
- As to why Leon Kennedy over other Capcom characters, I'll cite Sakurai's friendship with Hideki Kamiya, who had his directorial debut in the original Resident Evil 2
- I always find the last slot hardest to fill. I'll name two possibilities
- Lloyd Irving/Yuri Lowell/Velvet Crowe - based on the great relationship between Nintendo and Namco Bandai, meant to further drum up hype and sales in the East given that the Smash community over there has never been as large as it's been over here. Nintendo, wanting to change that, includes a Tales of character since the series is massively popular over there (it got the Xbox 360 to actually sell when Tales of Vesperia was an exclusive for it, and that's pretty notable given the Xbox 360 being dead before it even released in Japan)
- Nintendo promotional rep - If we're getting another one. Nintendo is naturally going to want to promote their own games in Smash Bros. As to who it could be, I propose
- Euden - to represent Nintendo Mobile and draw players into Dragalia Lost (this could be especially effective with a cheap deal like the Dream Summon they have going on right now).
- Astral Chain Twins (similar to Olimar and Alph) - To advertise and drum up interest for a new Nintendo IP
- Blyeth or Gen 8 Pokémon - Because we're all thinking it
I also believe we're getting another Ballot (since every Smash game has had a poll or ballot of some form), the 'winner' of which will be included in the game (a lá Sm4sh's Ballot). As to who that is, I don't know.
Please be Sora.