Current the game support 5 level: -2/-1/0/1/2
But in real world airport has far more than that:
Singapore Changi has 4-8 floor
Tokyo Haneda has 5-8 floors
Souel Inchoen has 5-6 floors
Hamad(Doha) has 3 floors
Hong Kong has 4 floors
so many large airports had floor more than 5 and the game need floors than 5
But for what do we need so many floors in ACEO?
The game currently has no lounges, no cinema, no cactus garden, no airspace control offices and whatever those airports provide in those many floors. 
Why would you need so many in the game? I don’t even use all of the available floors right now.
It’s not so difficult to fill all the levels, tbh… 0 for main hall, bus stands and arrivals, 1 for departures, 2 for big birds and offices whereas -1 is full with baggage system…
That said, adding extra floor is not such a big priority, I can live with these levels and I’d love to see other features first.
Adding more levels natively, means that more levels have to be supported in terms of performance. Of course, ideally I would want ACEO to have 100 levels up AND down, because why not. For performance reasons the levels have to be limited however, and the current 5 levels offer enough room to build creative airports.
IMHO the current “problem” is the size of the map (wayyy too small) and not the floors count, yes it would be nice to have maybe two more, but I think we can design nice and efficient airports with the current number of floors
I don’t need more floors now, maybe in the future.
However, I’d like to place parking space also in upper floors and put the tower on the topmost build floor of the terminal.
Will we see more level in a36?
I think it has already been said in the AMA that no new levels.