animation blending isn't that bad. If you have a two poses represented as lists of quaternions and positions, all you have to do slerp between the quaternions and lerp between the positions.
FABRIK IK algo is a ~100 loc function.
FABRIK IK algo is a ~100 loc function.
audio can also have unending scope if you want to do physically simulated Spatial Audio.
Im not sure if AI/pathfinding are worth developing as part of an engine. I feel like their implementation is heavily dependant on the game type, engine implementations often get in the way, rather than helping.
rendering is a beast, especially if you need a long draw distance and have a world that doesnt fit into gpu memory.
The whole task of putting all the pieces together into a cohesive package is a huge undertaking as well.