I thought about the following:
Currently one can build terminals to handle passengers. If a high quality building is constructed this will increase the Ratings for all Passengers that are beeing handled by the terminal. However it is rather expensive to build one (armortization takes over a year). If we look into reallife we find that only few airlines build their own terminals.
What most airlines do offer are Lounges.
Lounges are smaller areas within a terminal that offer a comfortable waiting environment for airline passengers. Often these Lounge are reserved to Premium Passengers.
So my idea would be to just rename "Terminals" into "Lounges" and offer an option to assign them to specific travelling classes (e.g. Business Class). By that one could really focus the impact of additional comfort to those passengers who really look for it(First and Business), as Economy Passengers often prefer a lower price over additional comfort. As Business and First only account for about 20% of the over all passenger demand, the lounges would take a much smaller investment and are therefor more affordable to build.