The Genesis allows up to 5 players, as well as Sega CD I believe (since it’s an add-on). There’s a surprising amount of great games that support 4-5 players. I’ve looked through the core options and it allows you to configure all 5 controllers. However, I can’t find the toggle to switch the Team Player or 4 Way multitap on or off.
Does the Genesis Plus GX core support multitap options?