Students tend to tell you they don't set any roles in the team when they work together in class tasks. That's something that dilutes with time, and probably the majority of professionals will tell you that you do need roles. At the same time, roles tend to alienate people and make jobs boring. So what's the solution?
We understand roles are necessary to maintain a clear structure in a team and organisation. But there is a way you can add some fun to it; experimenting with the creative process.
Ideation and brainstorming sessions allow you to have run some experiments with roles. There is no reason why you shouldn't rotate and change team member's position.
For example; you could set a different facilitator for each session (if you don't know what a facilitator is, check this blog post). In that way, everyone will have the opportunity to research on creative process and idea facilitation. Your team will also benefit from different exercises and methods.
You can also rotate perspectives. Have different members of your team taking the role of the client, user or creative. Make them think in someone else's shoes.
As an extra, you can set a different decision maker for each session. That person will have the last word on discussions and the power to make final decisions. Scary? You shouldn't! That will make your team members mature and gain confidence. Start with small projects and see how it evolves.
Ideation sessions should be dynamic and challenging. Fixed roles are the opposite of that. That's why, probably in your daily routine it would too messy to rotate roles, but when it comes to the creative process, you can.
Good luck with it and don't forget to have fun!