But if you have IK solver for your previous game lying around, you can do some goofy stuff like procedural animation for your (literally) butthurt character
Ok, let's talk IK. Usually you record your mocap animation on the flat surface and you can't just use it as is. In many games you play on the curved surface, and you need to take your animation, grab legs, glue feets to the ground and then solve bones backwards.
(Note the leg positions, btw). Nobody is actually doing it, cause you average player will not care. For example, Capcom in MHW did not even bother even though they had a budget of a small ethnostate