"custom" transitions means we'll be able to write our own transition functions? That seems like a lot of work! The examples I showed you would be enough [for me, at least], also their formulas are publicly available.
Personally, I'd prefer if you wrote this as a purely mathematical plugin that didn't attempt to move stuff. It could be only expressions and return values in the [0,1] range, though I concede that it wouldn't be as friendly for newbies.
The toggle to ignore movement is also a TODO item.
There is already the function plugin if you want to write pure math functions in JS.
PS: If you were to look in the code you'll see the transition functions aren't that scary. They just compute cosines using a bit of wizardry to translate the lerp fraction to an angle in radians.