oh we went through this with my daughter. however when she was little we had her room in Barbie but only the curtains, bedding, and the cling ons for the wall. We were renting so couldn't paint. When we bought this house she wanted a Bratz room but I wouldn't let her as she was 9 or 10 and knew that phase wasn't going to last much longer. We finally agreed on dolphins and I painted her room blue with a dolphin border, she has several dolphin nick nacks around the room, lamp, and we put decals on her ceiling fan. Now she is 15 and wanting something different but I haven't got the boys room done or the bathrooms and told her it probably won't be happening soon until the rest of the house projects are done. She does have a ton of various posters in her room too as she changes what she likes with changes of popular movies. She wanted a pirate room too and has just about every jack sparrow poster, batman, and now likes the mad hatter. posters are easy to remove and only has so much room for them so they get changed around often as she still likes some of the older ones and will switch them around when she gets tired of seeing one.
Pick a general color that would go with many themes and paint the walls, you can change the bedding every few years and boys are usually content with that because they can see their bedding and feel like that is the theme for their room.
I bought a sports border for the boys room but haven't had the time or energy to do their room but they have sports comforters and a few sports plaques/shelves and all their sports photos on the walls that they played in plus their trophies on shelves so they have several sports items in their room but never got the walls painted or border up. They have been content with saying they have a sports room without the extra work that I wanted to do with painting and a border.
Now they are talking about having a cammoflauge/hunting theme and they have some of that already in their room with their deer mount and deer skin plus some fishing pictures. I am considering this one because they are both into hunting and would be something they would probably like until they move out on their own.