Thanks all for the input so far!
I am going to work on Lounges first (just started some coding yesterday) and perhaps airport expansion later
For an invisible link connecting airport, yes i remember that idea and i actually did think of implementing it. I guess i just forgot about it and we have discussed many things before lol
I agree the profit margin will be thin no matter what as airlines competing for dominance, unless airlines put in some price agreement to avoid choking eachother out
I wouldn't want airline to just build lounge in every hub (probably not possible as it requires being top N passenger volume before one can be built) but i will think more of how to make it more strategically fun !