The team just has to come up with a solution. If it's theoretical that's ok, but they should have research, and present it in the skit or presentation, that shows why their idea is a valid one. You should have a notebook that includes the research the kids did, print out of emails with specialists and newspaper, magazine or other material that the team used to come to their solution. Include photos if you have them.
Our first year, the solution was purely theoretical, and the team won 1st place in research at regionals. The second year, they conceived of an idea that could have been made, but that they didn't make (because of expense) and won grand champions, and then last year, they made 4 different prototypes of their ideas :) and won 1st place in research at regionals again. So it doesn't seem to matter if they make it or not, as long as its an idea that is backed by research and info. It seemed to really help that my team always had a long list of professionals they conferred with through the research process. Most of the time we had email partners, but the last year we were able to use a lot local professionals to help them. It is more fun in person, but the info is just as good from specialist around the world.
The first year during "Body Wars", we spoke back and forth to a geneticist in England. My advice is, have the kids create the letter and then you e-mail it out to at least 15 people. Most of the people we've reached out to in the last 3 years have not written back. Only a few write back, so cast a wide net. Several of the professionals we've spoken to let me know they looked me up to see who was writing them so include your team facebook or team website if you have one.