Multilevel hierarchy Task board

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To organize our backlog (Planning Board) we are considering to group some items. We could do this by using a Product backlog items as a parent or introduce another workitem type. In a separate test project I added a workitem type called Theme.

This works perfect for the Planning Board, but the moment I move a Theme to a sprint and open the Task Board I run into the limit of the hierarchical options there. As far a I can see the limit is for only one level of parent child relations. So now the Task board shows me the all the items below a Theme, but not the parent child relation between them.

For this I see 3 possible solutions:
1. Extending the support for multiple levels in the Task board. I'm not so sure this is actually a good idea, since it may make the Task board to complex. I would however like to know if there are any plans in this direction?
2. Hiding workitem type from the Task board. Even if I leave out the configuration for the new workitem type from the configuration file I still get to see it. Would it be possible to make it configurable which workitem type I want to see on the Task board?
3. I could decide to use the Theme type only on the Planning board and not move it to a sprint as a whole. This we can do right now and is an approach we will probably choose at least for the short term.

Any advice on this would be welcome!