How big of a co-main does UFC 229 need to break the PPV record?

[Axl']

The co-main doesn't matter. Casuals don't give a shit about that, they just want to see the Conor fight.
I guarantee you that the buyrate would be the same even if they had stacked the card, and of course they are not going to do that because they're cheap and are already paying Conor shit tons of money.

 

[j18lee]

Wmma fight please Dana

Even though you're being facetious, your reply actually provides the best answer to OP question of what co-main would help UFC 229 break the PPV record.

McGregor brings in all the casual sports and fight fans. Khabib brings in the remaining hardcore/elitist fans. That's almost everyone who would buy PPVs, except....

Girl power casuals who bought Rousey fights in support of WMMA. This is probably the only possible MMA fans who would not be interested enough in McGregor and Khabib to buy the PPV. If they add in a women's big name title shot, that would give UFC 229 the best chance to break the PPV record. Unlikely, but if they could get Nunes to agree to rush her recovery to step in against Cyborg, the PPV record will fall.