In big airports like London Heathrow, Paris CDG, etc, there is a very limited slot availability. If you build a terminal in the airport, will that increase slot availability? Thanks for your help.
No, a terminal won't help.
Terminals only effect the rating of your flights and the amount you pay for ground handling.
Is there anyway to add slots to an airport? Do Airports ever increase their slot number?
No, you have to wait until other companies cancel flights or go bancrupt to free used slots. Or just find another spot in the world where you can start.
Watch out for big airlines with CCC ratings. They often go in a few weeks, giving way to a large number of slots.
Many others also watch them, so make sure you move in quickly.