If this happened, it would go completely counter to what Sakurai said in his last interview. He's looking to the ballot for inclusions in this game. On your list, Geno is basically the only one that would refer to for the base people. It would be strange to me that suddenly after saying that, he shows us a Pokemon that wasn't on the ballot, his own Retro character that's not on the ballot, Skull Kid who's far back on the ballot, or even someone who comes from a complete left field.
I suppose these things are possible going from past Smash games...but I would say that if there was a Retro character, it would be Sakurai's pet project this time and we'd have 5 in base instead of 4. He'd do it on the side to make sure it wouldn't take over tons of development time in someone who wasn't asked for. I seriously wouldn't undercut Isaac's chances here. I realize that there's a lot of antsy feelings as we get closer and closer to the final reveals but you can all rest assured that Isaac got his votes in this ballot.
It really all comes down to concentration of votes. If there were a household name character left, it would have changed things for general voting. Let's just say hypothetically Luigi were never in Smash all the way up til now. The ballot comes along and even if there's not a big vocal majority, he would get tons and tons of general votes from people. There are no more Luigi-like characters in this scenario that would suddenly appear from nowhere and crack a top ten. No Wario is left. The Ballot vote then falls on who drums up the most concentrated pockets of popularity. Realistically speaking for Smash, the only ways to do it is through social media and message boards like this. There's a reason that the person that compiled the exit poll data chose the specific polling places he did. Because that's where you get an understanding of how the snowballs start rolling into bigger snowballs.
This is why I said in a previous post that having a smaller sample size isn't too bad if it's representative. Isaac doesn't fall out of a top ten and is typically top 3 in those polls. The fact that we're seeing some of these other characters appear as ATs and Echoes is indicative of this. Shovel Knight wouldn't have been in if this wasn't the case. The only sort of character that might be surprising in a larger poll is someone like Simon Belmont who has history or is simply notable. Isaac has always been standing right behind Ridley and K. Rool, that's not just smashboards talking. I am constantly on youtube, gamefaqs, 4chan, reddit browsing this stuff and I was doing the same in 2015 and even from before Brawl. I'll verify it with a notary that Isaac wasn't a nobody back then during speculation.