Coffee filters that are pre-flavored are entering the market soon, if not already.
I have been unable to find any in the market as of this date, but I have found a US Patten application for flavored coffee and tea filters.
There is one problem I hope they find an answer for.
For those of us with poor eyesight and less than nimble fingers, separating those filters can be a real task. So the standing operating practice of just using how ever many your pre-coffee nerves can separate could be a problem.
Four or five pre-flavored filters could make that pot of coffee, well.......taste like......