Well if you watched the documentary included in Head On: Extra Twisted Edition you get some insight on what they want to do with the next title. Jaffe said he got bored with the dark, same color palette from Black so I think its safe to say this one will be a little more colorful. I'm sure the storyline and universe will still be pretty twisted and fucked up though.
Also, you can bet on there being all kinds of online modes and play as this is pretty much the standard in high quality games these days, as well as Jaffe being all about multiplayer.
My idea is that its going to be more sandbox like, ala Burnout Paradise. I'll bet there will be car customizations too so you can have an identity of your own.