One challenge we faced was to keep the interface simple and consistent.
During the design process we noticed several times that we had to change our prototype, because, for example, we did not offer clear exits or the user did not understood the menu items. But fortunately our test-users realized these mistakes and we could correct them.
Another challenge was to find a possibility how the user will interact with our system.
First we decided to built an entertainment display, where the user can chat and play a game. Afterwards we thought about the way how to interact. We resolved to take something what the users already knows and uses. Therefore we decided to use a Java application on the handy to interact with the large display. The menu structure is designed similar to the common menu structure on the mobile phone.
In the same way we attempted to play on the users experience and structured the chat as similar as possible to a standard sms menu.