[Update]: The Airports expansion has now arrived, and while it is a bit buggy, so far it looks like it delivered exactly what it said it would.
Many years have gone by since Cities: Skylines first appeared, but the developers don't seem to be slowing down even a tiny bit. In fact, they're currently preparing to release yet another new expansion on January 25th - Airports!
As the name would imply, the Airports DLC will bring with it the ability to design and construct your very own airports out of a variety of modular buildings, a more cohesive way of including cargo terminals, as well as an easier way of connecting your airports with your public transport services. And just to spice things up further, there will also be a bunch of new building options to choose from depending on what type of airport you created.
What this looks like in action, that you can get a brief peek at through the official announcement trailer. Have a gander, it's a fairly short one:
Perhaps the best thing about these types of DLCs is that they are so hyper-focused on one specific thing that you can instantly tell what you're getting yourself into. So if you care about airports you'll most likely have a blast with it, and if you don't, well, you're not going to miss out on anything too important by simply skipping it.
Whatever your thoughts on the matter may be, you can learn a tiny bit more about the Airports DLC, as well as keep an eye on any further developer updates, over at Steam.