JetBlue - Similar fleet and complementary networks. Can you imagine the disappointment at the legacies if a strong competitor could connect traffic in mid-continent and also run TATL from JFK on XLRs? JetBlue gains the west, and Frontier gains Europe. All with no widebodies and an efficient 2-type fleet! JetBlue needs some newer A320s too. Frontier would probably end up painted blue in this case.
Breeze - They have a mainline hub-avoidance model that also complements Frontier. There are times where smaller planes circumventing the legacies are a good thing, especially compared to a 1/3 empty bigger one. Fleet and network would mesh in the sunbelt cities, but their model is more about avoiding direct competition, rather than undercutting it.
Now if all three (F9,B6,MX) merged, that would create a force to be reckoned with by the big 4 and a competitive market for consumers coast-to-coast with 5 major airlines in the US. A fleet of 513 planes (279+174+60) with orders for 287 (98+151+38) modern and fuel efficient aircraft in just 2 types would make them a force to be reckoned with. Especially if they preserved and combined the strengths of all three - JetBlue's service and reputation, JFK, and TATL, Frontier's cost efficiency, Breeze's smaller-market focus and lower frequency schedules where needed. Such a combination would only really then leave new Alaska, new Allegiant, and Avelo besides the big players...
It's seems a large fleet, broad network, and diversified revenue stream are the requirements for survival these days in the industry...