How much work do you want to put into this project? How much will you use the finished project? Make something that pleases you and don't use precious time that might not be of value/use to you in the future. Could just use the color in a favorite design. Sometimes challenge pieces only fill the bill for the project, not our own desires!
There are a number of patterns using trees. Could maybe incorporate the tree idea into something for Christmas? My first thought though was something to celebrate St Patrick's Day, coming right around.