I'm pretty sure the reason they used Yellow and Blue Toad is so they could have the characters be red, yellow, green, and blue. With four-player simultaneous play you need to be able to tell which character is yours instantly, and having two red characters (i.e. Mario and Toad) or two green characters (i.e. Luigi and Yoshi) could confuse you for a fraction of a second long enough to kill you.
Whether that was just for the E3 playthrough or if it's going to stick through to the final version, I don't know. The fact that they've released official renders of
Blue Toad and
Yellow Toad, along with its release date being so close, certainly add an air of permanence to the whole thing. Still, I'd be pretty surprised if there weren't any unlockable characters.
Also, there's this, so you can't really stay upset at them.
I do wonder if Yoshis will have different abilities based on color or if it's just cosmetic this time around. I hope they do.
It is what happens. That game is a highly overlooked game, and it was fun.
Indeed.