there is a challenge I saw a while ago that I liked in a 20in20 comm for OUAT, and I was going to use in my comm at some stage in the future. But them theme basically is cast/recast, and I think it will work wonderfully here especially with everyone having different ideas on who the 12th Doctor should be.

You can ask everyone to make icons featuring different actors/actresses that they think will make a good future Doctor.