If you could start an airline...

Country: USA.

Main Idea: To become a "High Value Carrier," as opposed to a "Low Cost Carrier." Charge rates around what you'd pay on anyone else, but give you the things that the others don't: ability to bring a big suitcase for free, no fights for bin space and related gate lice issues, enough room in coach, not getting pwn3d if weather or some factor outside the airline's control cancels your flight, etc. Use phenomenally fuel efficient aircraft and have only one type of aircraft to minimize cost (also half my fleet would be at the FAA maximum 50 passengers per FA).

Fleet: Bombardier CSeries, CS100 and CS300. For two reasons. First, it has some significant fuel advantages, and if gas prices go back up in a few years that would be tremendous (especially considering "a few years" is the lead time on an aircraft order in the real world). Second, the fuselage is JUST wide enough where the seats are 18.5-19" wide (19" if you're in the sole middle seat on each row), but not wide enough for competitors to squeeze another column of seats in. So you can economically have the widest coach seats of any airline.

Route Network: Hubs Pittsburgh, St. Louis and Oakland. Focus city of Honolulu (yes CSeries can fly from HNL to as far east as DEN within fuel range!). Some point-to-point markets from the major Northeast airports and CLE, CVG, STL and DTW as these routes seem underserved and overpriced in real life. Also, from my HNL focus city I'd look at flying the Island Hopper HNL-MAJ-KWA-KSA-PNI-TKK-GUM, as UA charges outrageous fares on this route, which serves nations whose residents do not have much money, and the fuel range of a CSeries would cover all sectors. CS100s would be great at OAK as there are several runways at OAK that can handle a CS100 but nothing larger, so it would make the hub less congested.

Some specifics I'd do differently:

Baggage: Rather than charge a fee for the first checked bag (allowing overhead bin bags free) or the first overhead bin bag (like Spirit/NK and Frontier/F9 do), I'd simply say "First suitcase or duffel bag up to 50lbs is free. You can carry it on if it fits in the sizer, or check it for free if it's larger." This would create an incentive for the kettles (non-frequent-fliers vacationing who are most likely to not know what they're doing) to CHECK their bags, as their massive suitcase full of clothes would be checked and free. This would free up the overhead bin space AND SPEED UP BOARDING. With no overhead bin space issues, there should also be less of a "gate lice" problem at boarding.

Seating: 32-33" pitch but use slimline seats to create the same legroom as 34-35" regular seats, rather than using slimlines to cram a 30" pitch that claims to feel like 32". Combined with the wider seat design of the CSeries, this would lead to the most comfortable coach seating of any of the majors. The fuel advantage of the CSeries would ensure that I don't get forced into a higher density by competitors. Also, the CS100s would have 100 seats so only two FAs needed. Single cabin.

IFE: Bring your own device to stream free entertainment, wi-fi on board for purchase at a reasonable price. 110v and/or USB plugs for your device standard at every seat.

Food & Beverage: Two tier meal service: snacks and non-alcoholic drinks are free. Full premium meals (think premium cabin quality) would be available for purchase on longer flights, complimentary for premium cabin passengers (and passengers who reach a certain elite status level).

Other: All tickets but the cheapest value tickets (or tickets booked on credit cards which have Trip Interruption Protection) would come with trip insurance built in, so you would actually be covered for your cost of a hotel room and meals if weather or some other factor cancels or delays your flight overnight and you're out money. All seating at all gate areas would have at-seat power so you don't have to hover around a power pole to charge up pre-flight (not that you wouldn't have a plug at your seat on board).