It’s quite difficult to narrow down who is “most likely”. But I don’t think it’s accurate to say, “AnYtHiNg CaN hApPen1!” or that anything in particular is “going to happen”.
It’s quite interesting that something happens that wasn’t entirely expected, and suddenly nothing matters. I don’t necessarily think that’s true, but eh...to each their own.
There are very few ways they could disappoint me. But simply put, we don’t really have enough information to make a hard call on the last 5 characters.
Still think pick 1 is Spring Man. Not as confident in the “Hero” situation, but I absolutely do NOT think it should be ruled out.
I think the most likely characters are Spring Man, Crash, Doom Slayer, Rex & Pyra, Geno, and Heihachi.
Yes, that's pretty much my exact wishlist (only replace Geno with Shovel Knight), but I always call my lists "predicshlists" for a reason, as I try to keep my picks grounded.
But I might as well give my reasoning for each:
Spring Man - ARMS is confirmed as FP6, and Spring Man is by far the most likely pick from Nintendo.
Crash - During the development of Fighter's Pass Volume 1, Crash had the loudest fan demand, and I think Sakurai heard us loud and clear.
Doom Slayer - Same as Crash, but definitely second place. Sakurai has also name dropped John Carmack during interviews, and Doom is definitely the most important FPS game in gaming history. Master Chief may be the most popular, but Doom Slayer is to FPSs what Mario is to platformers, or games that don't repeat a few screens over and over where you just go for a high score in general.
Rex & Pyra - Should be obvious, but Sakurai outright said that Rex was his next pick for a character, and now that Pass 2 allows for post-2015 content, Rex & Pyra have their fair chance.
Geno - The lack of a Geno costume, as well as the still up-in-the-air Cacomallow leak, is simply too conspicuous to ignore. I included Geno in my list of Individual DLC characters outside of the Fighter's Pass, but Geno could easily come with a stage, music, and spirits.
Heihachi - Sakurai said during Smash 4's post-DLC cycle that he considered Heihachi but couldn't come up with a good moveset quickly enough. Now that we have Terry, Heihachi feels like a glaring omission. Also, Terry laid the precedent of "back-specials".