If Bitcoin Talk introduced a shop, and one of the items for "sale" was the right to display a signature, then signatures could become a privilege that had to be earned through responsible posting.
Credits could be removed for bad posting, and added automatically for responsible posts. Signature allowances could still be based on rank, and this would preserve the existing merit system.
And that would look a lot like adding a demerit feature and we know that won't come soon.
Creating an entirely new system on top on this one before it has failed or before trying to fix it in case it fails sounds like trying 10 pills at once.
Rather than all this, we could have a simpler solution, where you lose 1 merit for each activity period in which you display a signature.