Inactive players are bankrupted after a certain period. The majority of the aircrafts would be sent to the official brokers in the world, who will then place them in the aircraft market, for sales or for lease.
The premium gate search would be able to help you with that. With the multiple filters available, you could list the airports where you have free slots but are not currently served from the specific hub. What's more is that you could limit the distance, and the passenger capacity of the airports listed as well. Then, simply click the create route button for the desired airport, and you are all set. The premium gate search is available for 60 free tokens or 10 paid tokens for a period of 60 real days.