My airline is almost a year old. I have only one type of aircraft seating 120 passengers. From my medium sized hub, I am running out of destinations with appropriate demand and no competition.
In your opinion, which is better:
1) Continue with only one type of aircraft. Serve airports with no competition, but whose demand is only 80 or 90 passengers per day.
2) Enter routes to major airports with competition and where demand is already exceeded.
3) Bring in a second aircraft type that is smaller and able to serve lower-demand destinations that currently have no competition - even though this raises costs.
Serve 70+ pax routes with the 120-seater. Enter competed routes of course too (but no idea in trying to push others out of it, just wait for reasonable route image first and see what sales you are getting and if the route is worth it). Get regional planes for the small routes. ...everything works.
You'll find large turboprops make a lot of money, relative to the cash investment to obtain them so that might be a good way to go. Maybe not as much as a narrow body jet but I'd guess a 50 or 60 seat turboprop, properly scheduled will make 70% or so of the profit of a 120 seat jet. Quite a good way to use your money. I would go for something like that and it will open up a lot more routes for you too - the 40 to 70 seat ones you probably haven't even looked at yet. Running two fleets isn't that bad either - you've been very disciplined to get this far with one and that'll be why your airline is quite stable and doing well. The jets will work on those routes but so, long as you're going to add a fair number (10+, not 2 or 3, 20 or 30 would be even better) I'd go for a turboprop fleet. You didn't say what game you're playing so fuel costs and aircraft availability will vary. Depending on the year, I find the F50, ATP, Saab 2000, Antonov 148 and Xian MA60 are all money-spinners when used correctly. The larger ones (ATP and Saab, maybe others) have the advantage that you can even add C class seating if required). With standard seating, not HD, you can quite sucessfully fly any of these types on routes up to 800 miles or so (range permitting obviously). DO NOT buy small regional jets! (CRJ, E145) and don't go bother going for those 70 pax routes over 1000 miles.