Seems like a softshell jacket would be way to go. I have a Pearl Izumi Gavia as well as a Gore Phantom, and both keep me warm down to the teens with proper base gear, plus they will handle light rain. I haven't ridden in temps lower than 15 F so I can't vouch for their performance in colder temps. The Gore Phantom is particularly easy to ventilate because it has removable sleeves, and they can function as large pit vents if you just unzip the sleeves about halfway.