I am a student animator with programming skills and after going back to 3D from a 2D project I really missed my onion skinning. And I don't mean locator tracking. I mean like real shapes. So I looked into the Maya API to see what would be possible. What I am offering here is the result of a lot of desperate programming.
What this script does, is render an additional pass while maya updates the viewport that can be displayed later. I made a nice little introduction so you can see it in action.
For now only Maya 2017 is supported. 2018 might work and I am working on a version for 2016
If you like it, consider buying it for a few bucks. But it's free and also on github, if that's more to your liking.