These routes are either currently well served (SEA, PDX, DTW) or just odd PVD, JAX, OMA, MKE?
The well served routes are served by an airline with higher fares that caters to the business traveler (Delta) and an airline that isn’t low cost but I really have no idea what they are trying to be (Alaska). Frontier has a fair chance to do well in these markets by taking from the pax that just want to get from point a to point b.
Referring to the odd routes, I don’t have a clue about JAX or OMA but PVD might to ok by taking pax from the New England area.
I do feel that MKE will do well because the vast majority of people in Wisconsin fly out of MKE, the vast majority of people in Wisconsin are also low cost travelers. I have lived in Wisconsin for 11 years now and can confirm. The fares are cheaper or of MKE so people will drive from across the state to fly out of there. Low cost travelers and a low cost airline fits well.
The winters in Wisconsin are brutal and people tend to want to fly to warm places during the winter, this is why you see so many seasonal charters to Mexico during the winter.
One could assume the summers in Austin are brutal and people would be willing to flock to a cooler Wisconsin, Milwaukee is a nice city and is underrated by most. It’s airport acts like a third Chicago airport, taking pax from North Illinois.