So I'm writing a whole-ass thread about what it is that makes some multiplayer games more toxic than others, primarily in terms of game design.

I'll just say, upfront, the most important thing is community management. Doing that can negate everything I'm gonna talk about. You put the work in to cultivate a positive community, and it doesn't matter what design choices you make. Conversely, failing to manage a community can, and generally will, lead to toxic elements taking over even the best-designed game.