Aren't themes already built into the classes already though? There are feats that reward the use of specific powers or energy-based attacks. Even most builds are themed. Want to be a Fire Mage? Pick Wizard and choose Fire based powers and Fire complimentary feats.
So, admittedly we haven't seen a whole lot about themes yet, but from what I've seen I think it would be a cool route to go if they wanted to redo Al-Qadim...

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.