Would tags be global or would they exist within the scope of a project? (just thinking ahead about potential naming collisions for generic tags, such as API)

One thing I would really like to see is tags as part of a tree structure, so I could for example have a tag that is 'MyProject/Api/Articles', then be able to search and summarise my time logs at any granular level (i.e. total time that includes the MyProject/Api tag (and any children of it)).\

I appreciate their may be architectural challenges to solve those more complex queries, but I think that sort of flexibility would really sell the platform for me.