There are changes planned to the executor design for the Foxy release, you could voice your request (or an offer to help) to the people involved with that. The interest in a new design was to some extent kick-started by this discussion: SingleThreadedExecutor creates a high CPU overhead in ROS 2(which I was part of).
I don’t know exactly where the follow-up discussion is taking place, but if you feel like you have useful input to the discussion you could try contacting some people (I’m pretty sure wjwwood is involved).
Although, I’m also pretty sure they will see this thread 
so no immediate action is necessary. Just depends on how pro-active you want to be and how soon you will need this feature.