Pretzel Melts - apparently, Arby's also tried out a pretzel bun.
Turkey Smoked Gouda - this one seems easily doable since turkey is part of the regular menu and Gouda is currently available with this summer's Smokehouse Brisket Sandwich.
I've never had a croissant sandwich from Arby's, but I don't see why not.
The French Onion Roast Beef sounds pretty good to me.
Gyros are actually something Arby's has offered before, but maybe they're looking at bringing it back.