We recently received a free update for Cities: Skylines in the form of European buildings and maps to use them in. Thing is, these new buildings will only grow in the European-themed maps. They won't show up in the cities you were building before the update, or in maps with other themes.
Modders, of course, have found a way to eliminate that peculiar restriction. There are two mods you'll need, the first of which is European Building Unlocker, by modder BloodyPenguin, which unlocks all the new European buildings and props for use in any map you want. This includes the buildings that grow in zoned areas, as well as the ones you place, like the European-style police stations, colleges, and so on. It also allows you to use the original buildings on any of the new Euro maps.
The second mod is Building Themes by modder boformer. This mod adds a 'Theme' tab to your districts panel that lets you control which districts the residential, commercial, and industrial European buildings will grow in. If you don't want the European buildings sprouting up all over the place, now you can specify where you'd like to see them.
Essentially, you can now play with those cool European buildings in whatever city and map you want, and dictate where they'll appear. Plus, if you change your mind about a district, you can just switch the theme. If there are buildings already in the district, they won't vanish, but will change to match the new theme the next time they level up. Pretty cool.
I tried the mods out on an old city of mine that I haven't played with in the past few months, and it works great. I just laid down some new residential and commercial zones on the map, used the new European district theme, and waited. Sure enough, European buildings began to grow, and I was able to place specialty buildings like fire stations and schools as well.
The Building Theme mod is in beta, and the modder is planning to add a theme manager tool, to allow players to create their own theme that can be used with custom assets. You can subscribe to both the Building Themes and European Buildings Unlocker mods on the Steam Workshop.