Here are a couple c++ plugin ideas that i found may become useful in mocap cleanup and rigging.
---Stretch along curve plugin with weights
Say i've got an arm, leg, or snout and i want to squash and stretch it along a curve this node (probably deformer) can be used. Currently i started with a very simple idea that i'll post onto my git site soon.
--Retrieve frame to bind fk skeleton to mocap translate animation
I'm not sure this is necessary but currently i was needing to go through every frame of mocap looking for one where the limb (ex: arm) was in a "nice" default position for ik (fairly straight, with okay bends at knee/wrist etc). Maybe a plugin command could figure out the frame to use, by using some sort of minimization.
--Find rotates to get fk skeleton to better match a mocap animated frame
Example suppose the wrist is slight off of where the mocap wrist is, maybe a plugin can try out a bunch of rotates on the elbow (parent of wrist) to see how to get it close to mocap wrist, and repeat for all frames.
One reason why c++ plugin writing for maya is very useful here is that there can be lots and lots of frames. So say had 2000 frames of animation and rig has 200 controls each with 50 things to animate could be dealing with a loop as big as (2000 x 200 x 50 ~ 20 million per character) . Which is quite alot to ask MEL to do.