So, I didn't even look at Europe or the US, and I came up with the following list, all airports that I'm certain I could start in now and be very successful, and in some cases, build 500+ plane airlines:
Sharm El Sheikh
Rio de Janeiro
Also Sao Paulo, which is a little over 12 months away from opening, so both existing airlines will likely go there, leaving the current Sao Paulo airport near empty, with a huge amount of domestic demand.
Seoul, Narita or somewhere in Brazil are probably the best options on that list for really big growth. Narita or Sapporo are probably the easiest place to have a 50-100 plane, profitable airline. There are 20 airports there that should be reasonably easy places to get started, with minimal risk of being squashed, with plenty of slots available, with no really big airline HQed with you and therefore not a great deal of long-term competition, unless one of the small/new airlines also grows. There are options for different airlines. Shorthaul & domestic like Japan or Brazil. Isolated & longhaul like Lima or Pudahuel. Short-medium haul international, like any of the north african airports. There were also at least another 20 options for size-4 airports, or with slightly more competition, that I didn't make a proper note of. Like Auckland, Adelaide, Lagos, multiple airports on page 2 of Asia.
And there will be just as many choices in Europe/US. The airport finder pointed me to an empty palm beach.
The world is far from full.