I would love to see self balancing Binary Search Tree data structure with generics support. Ready to start working on it as long as it is OK. Here I have a draft version written for my own library. Should be changed to match the overall standards of Collections-C repository. My AVL tree implementation:
https://github.com/khasanovasad/TheAlgorithms/blob/main/data-structures/include/ue_avl.h
https://github.com/khasanovasad/TheAlgorithms/blob/main/data-structures/src/ue_avl.c