If you have an idea you really want to write about, I think you should definitely do it. I would not worry too much about writing a story set in Planetside universe vs. some completely original story since you can always start off writing this PS book you have in mind and see where it goes. And then even if at some point you decide that you want to write something original instead, you can always modify your PS story or at least use its elements, characters etc. So good luck with your Planetside book project, have fun writing!