There is a new DLC coming out (Mass Transit). It doesn't seem like this game is very popular for some reason on this forum (or at least very active. Last post was over 120 days ago). According to steam charts though, the game currently has nearly 7,000 people playing and there are still always lots of new mods and stuff.
I actually just started playing yesterday, despite buying it way back in December. I kinda forgot about it to be honest, but mostly because I got super busy with real life stuff and I got into WoW with a real life friend of mine and WoW takes a lot of time lol.
Here is the DLC page
http://store.steampowered.com/app/547502/(and here are description+features, copy/pasted from steam store page)
Mass Transit is the latest expansion for Cities: Skylines, the award-winning city-building game, and it brings several new systems into play to help citizens traverse their towns in speed and style. Commuters can now get around on the water, up high mountain peaks, and even through the sky thanks to the addition of ferries, monorails, cable cars, and blimps to the game.
Along with new transit service buildings, mass transit hubs where lines can exchange passengers, new scenarios, new landmarks, and new road types, urban planners now have more options than ever to add unique style and personality to their growing cities.
Main Features:
Sick Transit, Glorious Money:
Add a wealth of new transit options to your city, and add wealth to your city’s income! Let your citizens get across town in ferries, blimps, cable cars, and monorails!
Orderly Hub, Bub:
New mass transit hubs can bring all of your services together, letting citizens change rail lines in one building, or hop from the bus onto the ferry, or even find their way through a sprawling monorail-train-metro station.
Become a Roads Scholar:
Explore a set of new challenge scenarios focused on fixing traffic problems and adding new transit systems. New road types, bridges and canals adds variety to your city, and new ways to solve its challenges. Become an expert in traffic flow, and then use that knowledge to improve your personal city!
New Hats for Chirper:
NEW. HATS. FOR CHIRPER. (and also new unique buildings, policies and achievements. But HATS!)