Most films for cinema are not in 16:9 format. If I shot a movie in 16:9 format in Ultra HD, would there be any technical issue with getting it played in theatres?
AS to source material:
As of 2009, the most common acquisition medium for digitally projected features is 35 mm film scanned and processed at 2K (2048×1080) or 4K (4096×2160) via digital intermediate. Most digital features to date have been shot at 1920×1080 HD resolution using cameras such as the Sony CineAlta, Panavision Genesis, or Thomson Viper. Cameras such as the Arri Alexa can capture 2K resolution images, the Red Digital Cinema Camera Company's Red One and Red Scarlet can record 4K, the Red Epic 5K, and Sony's F65 CineAlta camera can record 4K down-sampled from 8K. The marketshare of 2K projection in digital cinemas is over 98%. Development of the 4K Dalsa Origin was terminated by Dalsa Corporation in 2008.
So recording in 16:9 is not gonna be a problem.
When all of the sound, picture, and data elements of a production have been completed, they may be assembled into a Digital Cinema Distribution Master (DCDM) which contains all of the digital material needed for projection. The images and sound are then compressed, encrypted, and packaged to form the Digital Cinema Package (DCP).
For technical details, see this link for an overview: