What specialization to focus on?

It depends on what you are trying to achieve.. Do you want to get best population boost per Golden Key? I see no point in that, just buy what you like and what will make your city looks nicer, more interesting. 

I do not recommend Gambling - Casino buildings looks boring, especially the luxury ones. But get at least one to unlock special skyscrappers in gambling area.

You can buy Transportation - Ballon Park and Airship Hangar - so you will have Ballon and Air Ship flying near your city. Do not buy Heliport, there is no heli in the game.

I would also recommend Entertainment - there are most interesting buildings like Expo Center, Stadium and Sydeny Opera House. Also Feris Wheel is nice - because it's whirling 🙂

And of course get Landmarks - Empire State Building - it's the tallest building in the game and looks nice.

What ranj said.. If you want a nice city to look on, buy a little bit of everything.

If you want to maximize boosts, cover your whole city with pairs of universities, heliports and luxurious cassino towers.

I've 80% of the map unlocked. For the 50%, I didn't know about that and bought all kinds of things. For the rest of the city, I'll go with the enhanced mode :P