Something I'd like to see is a flag customistation ability, so that new kingdoms can have unique flags, or we can change the look of old ones to be more accurate (for example if you form the kingdom of England as a viking, you should not have the three lions as your flag). I'm not sure how old any of the Cornish flags in existence are, so it's probable that the most ancient variants are lost to history. Custom flags would allow us to develop something which we think might have suited the time (pre-duchy).
That would'nt be hard to implement. The title would be by default its historical CoA, but you could customize it using the same CoA generator as the one for your dynasty coat of arms.
It could be added to the Customization Pack DLC.