I'd like to see the return of the Monk, but patterned more after the Defender class from Midnight: less Shaolin, more "guy who can kill you with his bare hands, or some handy implement for those really tough jobs". Think SAS training. (Admittedly, some options for the Rogue come close ... but not close enough.)
I'd also like to see all niches for all Power Sources filled, particularly the Martial power source. What a Martial Controller would look like, I don't know ... the Grenadier? The Flamethrower? The Warjack Pilot? Maybe some sort of steampunk engineer type.
I'll second a Mentalist class, especially over the way psionics has been done before, as an alternate magic system; the new Powers-based approach shows promise. I'd also like a clearer distinction between telepathy/esper/precog powers, metabolic powers, and telekinetic/metacreation powers; those might even be the basis for three new classes, in the Controller, Defender, and Striker categories respectively.
"On two occasions I have been asked [by members of Parliament], 'Pray, Mr. Babbage, if you put into the machine wrong figures, will the right answers come out?' I am not able rightly to apprehend the kind of confusion of ideas that could provoke such a question."
- Charles Babbage (1791 - 1871)