Here's my take on Isaac (since I bet you're all dying to know xP)
As far as the ballot goes - well, as far as Nintendo characters on the ballot go, he's probably done pretty well. The peak activity the ballot will see has already been seen, it happened about the two weeks after it was announced, and in that time Isaac really shot up in popularity. So if we get three or more characters from the ballot, I'd say Isaac has a pretty good chance. That is, if Nintendo just wants to look at the ballot results, because externally, among the wider audience, Isaac has decreased in prominence from what he once was (and to those that say GS has always been niche - that is absolutely untrue. Now it is, I can't deny that, but when the first two were current it was right up there with Nintendo's b-tier series, even outpacing many series already on the roster).
If Nintendo were to look outside their ballot results when determining ballot characters, I feel Isaac would fall a few spots, because as it is, he is mostly appeasing the hardcore. That's not to say Nintendo will shy away from solely appeasing the hardcore, I mean they did add Roy - a character who was certainly not added to service those interested in Nintendo's current offerings, but rather to gratify longtime fans who have stuck with the series for over a decade. The question is just to what extent Nintendo will look outside the ballot when deciding ballot characters.
Due to factors beyond the ballot itself I wouldn't necessarily classify Isaac as the next following K. Rool regarding ballot characters (though I don't have one particular character to rank above him), and therefore wouldn't necessarily expect him to be the second character chosen of the ballot, more just acknowledging the distinct possibility that it could happen. I definitely could see it, but it also wouldn't surprise me if another were chosen above him (not counting K. Rool). At three or four inclusions from the ballot, I would bank on him, but I'd still hardly classify it a guarantee.
That said, this ballot is probably the best chance he has going forward, as I don't see a GS4 as a particular likelihood. At least not prior to his inclusion. If he were to be included, I do see a re-ignited interest in the series, which I think it definitely deserves. One sub-par game (when compared to the first two) should be little reason to drop a series. Almost every Nintendo series has experienced a sub-par game (when compared to the others).
Lastly, if Nintendo is looking to the future with this ballot, as in omitting characters better used as headliners of Smash 5 (such as Inklings), I do think it bodes well for Isaac (and K. Rool) as they are characters that are highly popular, and would definitely satisfy fans, but don't exactly have bright futures in their own series, so wouldn't clash with Smash's newfound bent on recency in their main rosters.