Sorry if this has been discussed before, but I did a search and found nothing.
I ran into a display at CUN yesterday about a new terminal, and it had a diagram, with a new, parallel runway on the northwest side of the existing complex, with the new terminal toward the southeast side of the complex, further east than the existing domestic gates. Here is the press release from ASUR:
ASUR's press release
Click on New Terminal 3 in Cancun
• An advanced security system that screens 100% of the baggage transported in every aeroplane, and which complies with the new international aeronautical security regulations
• A total area of more than 40,000 m2
• 84 check-in counters
• 11 boarding gates with boarding bridges, and another 4 for remote stands
new runway will be 2,400 meters long x 45 meters wide
This will ensure that Cancun will be able to keep up with their tourism industry, and I'm sure will be a good investment to support the growing economy.