If building a distributed KV store interests you, you might want to check out Georgia Tech's OMSCS Distributed Computing course: https://omscs.gatech.edu/cs-7210-distributed-computing . While rewarding, this class was hell and still gives me nightmares....
Going through this class now, and I can attest that it is in fact tough as hell. However, I already feel as if it will be the most rewarding learning experience in the entire program.