You can divide the debate into several topics and let the candidates choose for which topics they’d like to be included in the debate. Something like 5 candidates per topic is a bit more doable but obviously it would still not be as easy to follow as a debate between 2 candidates.

I'd set it up as a death match where they are all dropped into different parts of an island and have to survive with what they can find

Instead of news organizations holding general debates, I'd like to see non-political non-profits hold more narrowly focused debates. Can be broadcast by any channel that wants to air it.

Focus on two or three core, interconnected, issues with a few offshoots that are tangently connected.

Invite top thinkers on those specific issues along with the top presidential candidates.

Even if the other invites don't have a shot at being president, it will still force the presidential candidates to try to make valid arguments against or side with people that really know about the issues. The people really should hear these views instead of main candidates just trying to bring their opponent down. And if someone not in the top two expresses something that really resonates with the audience, maybe it gets picked up by a main candidate.