Honestly, I really think Nintendo would go all out with these 5 characters (one of which is Joker).
I believe that the next 4 characters are going to be:
1. A Square Enix rep; mainly Sora from Kingdom Hearts or Noctus from FFXV
2. A Microsoft rep; Banjoo or Steve for sure
3. A long requested character; Geno being number 1 in my book
4. Another Fire Emblem character; from 3 houses
What I believe they may do post DLC is release free updates to include more echo fighters in the game, if not free than $2.99, I'm talking people like Inkling, Birdo, Dry Bowser, Black Bull, Ninten, and ,my wildest prediction, promote Shadow from assist trophy to echo fighter, same with Rodin.
They could also release new spirit battles and online modes as part of a free update or something, but these are just my personal predictions...