In Ubuntu, a bug reported about a package in Universe may be of critical importance for people who have installed that package, but of low importance for Ubuntu as a whole, because hardly anyone installs the package. Unfortunately, Launchpad requires these two to be represented by a single Importance value.
This is causing difficulty for the QA team: either release management is made more difficult because bugs are marked Critical or High when they are not important for an Ubuntu release, or QA is made difficult for rarely-used packages because they are only ever allowed to use Low importance (or Low and Medium, depending on the package).
This could be fixed by letting bugs have a separate Importance value for the distribution as a whole, and for individual packages in that distribution.
I think this is actually a wontfix. If the bug is critical in universe, then it has to fit into the definition of critical -- and a bug in a package which is broken for only a very few people is probably not one.
It seems to me that you're suggesting that the collective of bugs ordered by importance is actually split into two separate groups: main and universe. I would agree with that, but suggest that fixing that is done by allowing people to view those groups separately, not by adding more knobs to twist.
I don't really understand "low importance for Ubuntu as a whole" -- universe and main are both Ubuntu proper.