I was afraid off that, I think you are doing a good job and I like the trello.
But such an infected topic as balance there is always a risk, just by the mere fact that the grading is subjective this might cause a drop in confidence for the tracker.
Or that the subject grading is used to disgard the Issue tracker.
If you don't trust the issue tracker to give an accurate depiction of balance issues in the game then utilize it only to monitor bugs, or not at all. If your concern is that relic will not implement changes or fixes based on it, I assure you that they don't and never will so that is irrelevant. It is purely a useful source of information for the community that relic may take an interest in. If you don't view balance issues as useful information, filter those cards out.
Its also really hard to find something concrecte and objective instead. posts on a thread is not going to work, an obvious majot issue will have very few posts because there will be a lack of discussion, a minor issue might have a tonne of posts just because someone derailed it.
This really doesn't even matter that much. There are multiple columns because it would be stupid to put all the cards in one huge column. Should I rename them to issues, more issues, and even more issues? If it's unclear which column a card belongs in then just leave it where it is.
Might there be the ability to add a vote also for the type of issue? So atleast the subjectivity is spread to more individuals? But maybe that isn't the way to go either. Im just trying to brianstorm here.
No, this is not possible.