Is there a way to set a default for planned for and file against based on Workitem Type?

I am putting this in my follow up pile to review. But I don't understand one point "But this might vary depending on which workitem type is open." Would it be reasonable to say you want to be able to set which timeline a work item is associated with based on category? And that this should only happen when the category is changed? I don't how to do this but I want to make sure I am answering the right question before I did in to it.

We have Workitem type of Request, and WorkItem type of Defect for example.   If a Request gets open,  I want this to default "planned for" to a timeline I have called 'Requirements Management Timeline'.   If the request is accepted, we will move it to a given release that it will be assigned to .   Otherwise.. i don't want the person who opens the request to by mistake drop it in to a release.

Then for defect, similar but I would like to put these when first get open to have a planned for of "Defect Management Timeline'  then later assign them to a release timeline once approved.   Ideally I would just hide the planned for field when the workitems are created. 

Chris

Understand, but my issue still is an issue in your example.  In that I don't want someone to enter the workitem and put it on set the planned for on an iteration I don't want it to be on yet.   As for the Filed Against,  I really want to have similar default.

I have a default set of catagories.   It appears maybe you can have different one for different timelines? What do you mean by "  it introduces additional complexity with categories"