Yes, the current terminal may be adequate for now but I think Croatia is looking towards the future, with respect to building a new terminal. Where do you get your info. about it not being used to even half-capacity? I visited a few summers ago and the morning I flew out, there was a Malyasian 777 to Vienna, a Lufthansa A320, Croatian A320, and a British 737 believe me the terminal was full, there were no seats in the waiting area as we waited for the bus to go to the tarmac. But that' why I say it's adequate, it's not that busy all the time, but occassionally, and especially during the summer months, it does get busy. Furthermore, Air France, Swiss, KLM, LOT, and more have all added routes since I was there last, so I'm sure it does get busy throughout the week also. Anyway, Croatia will likely join the EU in the coming years and as such, will probably see an increase in air traffic in Zagreb as well as coastal cities. Additionally, some carriers are already operating wide-bodies there (Malayasian 777s) and Emirates also plans to fly to Zagreb soon. I'm guessing much of the renovation/construction will center on building jetways and increasing terminal capacity to accomodate more passengers during check-in and waiting.
Backhanding the left into submission, one activist judge at a time.