You could just as easily stack a couple cards a year with 2-3 title fights and probably get the same buy rate. At some point, it doesn't matter how many more big fights you put on a card - you're already reaching all the market. I really don't think if someone isn't buying a PPV that has three title fights on it already that adding another two or three will do any good.
I'd love to see another UFC 100 type PPV, one with two BIG title fights (not just Jones vs. Vitor type title fights) and a couple big grudge matches.
Imagine a PPV like this:
Georges St-Pierre vs. Top Contender (let's say Ellenberger or Hendricks) - WW Title
Cain Velasquez vs. Alistair Overeem (if Overeem beats Bigfoot) - HW Title
Jon Jones vs. Daniel Cormier (catchweight bout)
Add two or three other good fights on there, fights involving top 10-15 opponents going at it, and you'd probably get around 1 million buys.
Or imagine a card that features GSP vs. Anderson Silva or Anderson Silva vs. Jon Jones at a catchweight bout and has another title fight, like a Lightweight or Featherweight bout...
I don't think you could add any fights to those 3-4 fights that would get the card anymore buys than it would already get.