I like the "Open Fight Night", except everyone would be calling out the champ. I suppose the condition is that whoever you call out has to actually accept.
If I posted the card for our "Open Fight Night" on a Wednesday, let's say, then that could leave enough time for everyone to challenge whomever until everyone has accepted a match and has a place on the card.
I also like the cumulative point total over, say, a two month span. And whoever scores the most at the end of the 8 weeks or so is given a World Title shot at the following PPV.
It's basically the same thing I do when I run the occasional "Invitational" here and there, but it would last longer.