I would like to have a mindmap in Basecamp to order ToDos and to visualize structures and hierarchies.
http://www.mindomo.com does that already but I don’t want to use diffent webservices for one project.

I don’t think they will/should do it. Not that I don’t think it wouldn’t be useful… I just see so many other things so far ahead in priority-order

We don’t have plans to add mindmap-like charts or diagrams to Basecamp, sorry.