Just to preface, these suggestions are kind of out there. They dont take the codebase into consideration at all but I think some good ideas can be extracted out of there.
1. "Delta Connections"-like airlines. Instead of people making alts, they can instead create a limited amount of regional subsidiary airlines (1 per continent/region perhaps, or limited in some other way). Maybe they can only make flights out of the main airline's bases in the region, and could cost loads of $$$ to start up. They could also be limited in aircraft size and maybe route size (length/throughput).
2. "Seasonal" Historical Competition . For like 3 months, everyone runs their airlines throughout history. As time ticks on planes change and better stuff becomes available. Start sometime in the 30s and end in modern times. That world (separate from the normal game) resets every 3 months (or some other decent length of time) and people who at the top of the leaderboards get a little title or badge next to their name until the next winners. If they manage to go bankrupt, maybe they continue on but arent eligible for those titles. Perhaps they go to a world separate from the main competition so they can continue the historical fun without affecting the results after they bankrupt.
3. Crowd Source route simulation - Similar to how Folding @ Home and others do it, distribute a client that people can run so that they can contribute a bit to doing all the route calculations (this assumes that this can actually be distributed, I didnt take too much of a look at the pathfinding code). People can compete on a "routes calculated" leaderboard. Something like this can help support crazy stuff like I suggested above without totally killing the server.