OK. Let's go.
Right now, the icon theory is sketchy. The only thing it ensures is the return of Mr. Game and Watch.
Icon number 19? I think it could be anything.
Will the icons go over 20? I bet you my shoes, they will. Remember when some people thought they wouldn't go over 18?
Will there be another third party franchise? Sakurai said, 1 or 2, BESIDES Snake. We already have the 1, can we get the "or 2"? With so many candidates, it IS possible, but it's also possible there is not another one. 50/50 on this one, but I'm hoping for another one.
Now, why thinking there is only one more unrevealed franchise is silly:
Hidden characters. More likely than not, some franchises will be totally hidden. Fire Emblem was, GAW was, and it even had a stage. Etc.
And the third third party is more likely NOT Tails. Dude, the fact that it would be easy to get him doesn't make him likely at all. *IF* Nintendo wants another third party, they can and WILL get it. And guess what? I wouldn't bet it's unlikely that there are more developers like Kojima, who would like their creations in Smash. There will either be no more third parties or a third party that is NOT Tails.