Anything related to multi-agent reinforcement learning. Popular frameworks and tools include PettingZoo, RLLib, Melting Pot, Mava, OpenSpiel, Tianshou, PyMARL and more.
Multi-agent reinforcement learning (MARL) is a sub-field of reinforcement learning. It focuses on studying the behavior of multiple agents that coexist in a shared environment. Each agent is motivated by its own rewards, and does actions to advance its own interests; these interests may be opposed to the interests of other agents, resulting in complex group dynamics.