There are a couple of 4-directional games that are starting to look good. They have put a ton of work into dealing with foreshortening and I usually see them do it by scaling bones up and down int he legs when walking "Up" or away from the camera.
On the Unity side of things, I posted 4-way/8-way movement something here on how to control it, but its just a bunch of screen bakes from a 3D model because I don't have the time to build a character like that heh. I'll reach out to some of the folks I know are doing the perspective chars and see if they'd be willing to post some of their progress π
In the mean time you should check out Game Character Hub too!
https://www.youtube.com/watch?v=u2XNHPnehG0
It's available on Humble Bundle right now for like $10-12 along with a lot of other stuff. They have some really cool iso-handling stuff built in as well as some great character templates to get you started.
My personal feelings toward iso animation is that if you REALLY need to do interpolation its almost better to animate a base character in 3D and bake out some pre-viz then put it in the background of your Spine file... I honestly don't know of a "best" way to animate iso characters with 2D tools. FFT used interpolation on individual parts to do things like make arms bounce up and down during an idle, but not to animate them rotating around the shoulder - they used image swap when that was needed. I would absolutely love to make a "Spine FFT" template at some point π But it'll be a while heh.