I don't think a Ganon Final is that likely to be honest. With Giga Bowser, he's just a bigger, badder, meaner version of Bowser, but most of Ganon's forms are much more distorted from Ganondorfs figure (especially if we are looking at the TP model of Ganondorf, which is the likeliest, who practically transforms into a different species). I don't think they are going to make a whole new moveset for a character you are only going to play for about minute each match, which I feel most forms of Ganon would need.
And honestly, they shouldn't overkill transformation Finals, and if Sonic gets in, I feel he needs Super Sonic more than Ganondorf needs Ganon (though Super Sonic would probably work a bit like a mix between Giga Bowser and Volt tackle)