I am just wondering, after the addition of SLC what could be some new routes. With the AA/US merger would CLT, PHL, and PHX be possibilities. Also since F9 is phasing out their E190s would they use A319s or could AA/US replace them on the DCA route.
Thanks in advance! 
I doubt that F9 will continue the DCA route. It doesnāt fit their new ULCC business profile. They will likely retain DEN and MCO on Airbus aircraft. MCO may go year-round. Despite there probably being a market for Madison-Phoenix, I donāt see the new American adding it as it would require mainline aircraft and major airlines just donāt jump into new routes on mainline aircraft very often anymore. I think the most likely addition would be PHL. If F9 were to drop MSN-DCA itās feasible that US/AA would add it, probably on a CRJ-700 or E-Jet, depending on how many DCA slots they still have after the merger. CLT is a possibility, itās advantage is that the only current service from MSN to the southeast is on Delta to ATL, but the disadvantage is that itās probably a smaller local market and has less international connectivity possibilities.
Any new airlines?
Allegiant could return, possibly with service to IWA and LAS and/or PIE. Itās one of the holes on their route map and I donāt think anyone would challenge them. However, they probably get quite a few people from the Madison area on RFD-IWA/LAS/PIE. If F9 were to drop MSN-MCO I can see them trying SFB as well.
I donāt think Spirit, Sun Country, Virgin America, Southwest, or JetBlue would be interested, and thatās all the airlines that are left.
Any Cancun-like charters possible? Again, though, you run into RFD and of course MKE.
Does Madison have a FIS? (somewhere for International arrivals to come in to). If not then no CUN flights.
Not necessarily. The airline could have a ātechnicalā stop enroute in order to clear customs. It could also be possible that MSN and another city with FIS are flown as one flight with the second city being the FIS point.
Yes, they could, but Iām not aware of any airline that currently runs such flights. MSN would have to be a very attractive market for an airline to do something like that.
Domestically, Casino Express (think itās called Xtra now) operates casino charters that serve two cities on each rotation to EKO/ENV. Same thing for Allegiant charters (not scheduled flights).
The Xtra Airways flights donāt usually serve two cities. They usually drop off pax at one city, then fly to another city to pick pax up. Then do the same thing at the end of the trip. For example, one week theyād do something like IFP-STC, drop pax off, then ferry STC-FAR, pick up pax, and fly them to IFP. This is because the charters operate to different cities each week.