I suspect that small flash technology with the 600RT has reached a relative peak and is leveling off. It will be difficult to get more photons out of it as that's fighting physics. The wireless technology isn't going to change, again physics and band regulation. That just leaves firmware and features. At some point they might go to a color touchscreen which wouldn't hurt, of course that would eat more battery too.
Maybe they can make a flash with adjustable light temperature.
That's what the supplied gel holder's for. Mine came with a few gells too.