It could be argued that Frontier is in the process of doing that - at Cancun.
Once services from Nashville and Austin start, Cancun will have more Frontier destinations than their designated focus city, LAX
. And if - stress "if" - they get the rights to STL
, Frontier will have as many routes to Cancun as any US airline (AA has the most with six).
Cancun can never be a true focus city, of course, because I doubt Mexico would ever allow Frontier to have onward rights.
But is is interesting. And different.