I just feel like Isaac has a shot next game because with Ultimate's small cast of base game newcomers due to Everyone Is Here that were all extensions of already present series that were popular options (Ridley, K. Rool, Dark Samus, Chrom), more contemporary characters (Isabelle, Incineroar, Inkling), or Castlevania, Isaac doesn't really fit--like imagine getting four new stages and it's New Donk City, Great Plateau Tower, Dracula's Castle, and like Venus Lighthouse, he kinda sticks out like a sore thumb in that regard--nor did he fit into the DLC where everything was third party or more contemporary (Pyra/Mythra, Min Min, Byleth).
In a game with a more standard newcomer number, I could see Isaac being acknowledged as a popular request via the content in Ultimate being a potential point in his favor, though it obviously depends on a lot of factors, namely if the next game actually has a more typical newcomer pool and if that consideration continues into the next game.
So like I don't think he's a lock or anything and he's not even one of my most wanted, I'm just like "Wow, Isaac actually might have a non-zero chance for next game" lol