It would be nice if it was not possible to close an issue without setting a
resolution. This would prevent a common oversight.

If we do this I think we should add a new resolution "redirected to
python-ideas".  I have been leaving the resolution blank when that's
what I've done, since none of the other choices seem appropriate.

Wouldn't "rejected" be OK in this case?
A new idea could either be accepted or rejected, and I don't think it's a big deal if we leave the issue opened for a few days while python-ideas takes a decision.

Does Roundup support a concept of dependencies between field, or would this have to be added in an ad-hoc fashion?  For example, another useful modification would be to move from “patch needed” to “patch review” when a patch is added.

It would have to be added, but it is not ad-hoc, it is completely programmable.  Python code can be hooked in before or after the field value changes are finalized, and implement whatever dependencies we want.  This is how autonosy and email notifications work, for example.

vis the "patch review" change, though, note that a patch-like thing being attached to an issue does not always mean it is time for a patch review.  The fact that it is most of the time might be reason enough to do it, though.

detector/patches.py is the automatically adds the patches keyword; it could easily make other changes also.

Is this feature still desired?

FWIW I recently redirected an issue on python-ideas using the "rejected" resolution[0].

[0]: http://bugs.python.org/issue12635