There are definitely opportunities out there for another focus city. A few years back Midwest was looking at establishing another base of operations outside of MKE and MCI but they never got that far due to obvious reasons.

For speculation, here is my list of 10 cities where Frontier might have an opportunity to create a small focus city/connecting operation. Any attempt at another focus city would require Frontier to dedicate time and resources (aircraft, money for proper marketing, etc.) and should only be undertaken once existing opportunities in DEN, MKE, and MCI have been maximized.

1) Sacramento
2) Austin
3) Pittsburgh
4) Richmond
5) Greeneville/Spartanburg
6) Birmingham
7) Jacksonville
8) Ft. Myers
9) Columbus
10) Portland (PDX)

Hartford and Raleigh would have been on my list but since Frontier recently pulled the plug on both I decided to remove them.

Each has their pros/cons. For example, on paper AUS looks like it would be a very desirable focus city but it has been a tough nut to crack for airlines.

I do think we'll see Frontier begin to focus more on Mexico and the Caribbean in the future. However, there are still good opportunities to exploit in DEN and MCI. Frontier will also have to decide what role MKE will play in the future and just how aggressively they will pursue certain markets.