I agree with aerosnake. Whole ploppable airports are boring and typically ugly. Here are some thoughts on airports I would love to see.
Runways should be placed like roads
-they should be draggable
-they should have different sizes to accomodate different aircraft
-taxiways should also be placed like roads and connect/criss-cross runways for efficient taxiing
-length of runway determines aircraft
Terminals and Concourses
-You should be able to select a visual style among a variety from around the world
-Create terminals by click and draging the building to make different terminal/concourse designs. (also similar to road building mechanism but creates 3d buildings instead of 2d roads)
-should allow us to create curved terminal (MIA), satellite concourses, (ATL), etc. for example,
http://en.wikipedia.org/wiki/Airport_terminal#mediaviewer/File:Terminal-Configurations.png
-Allow for multi-story terminals and concourses that allow for higher capacity
-tarmac around terminal/concourses should use the same "paint/fill" tools that zones uses
-options to build train/subway/peoplemover to connect terminals
Hangars, control towers, fuel depots can still be ploppable, perhaps with different size options
Ground transportation
-should be able to use most of the road tools to build the ground transportation network for the airport
-parking (ground lots or multi-story decks) with different designs to match airport style
-park and rides, connection to highways
-"zone" taxi stands and bus terminals
-connect rail/subway station to airport
Other options
-onsite hotels and convention centers
-onsite fire station
-freight terminal
-rental car parking lot or deck
airport system should have its own mini-economy (without becoming an airport simulator)
-your city's size, wealth, economy, and tourist attractions determines basic demand for air travel services.
-this demand equates to demand for specific number of gates (determined by terminal/concourse size), and runways (terminals/concourses + runways = capacity)
-precisely meeting demand for those items makes your airport run at an average level
-capacity below demand results in bad airport grade
(delays, cancellation issues etc.),
-capacity above results in high grades, (happy travelers, tourists, potential for airlines to build hubs to further boost local economy)