Change Astalor the Flamebringer’s ability so that the damage added from manathirst is separate from the battlecry. For a normal player, he still deals 14 damage, same as before, but with a Brann in effect, he’d only deal 21 damage instead of 28. There are cards other than Brann that double battlecries, but Brann-Astalor has been the most prevalent. It was a problem before, and now there’s a new Brann so it’s a problem again. It would still be a powerful combo, just less powerful.
Yes, I know this doesn’t solve ALL the problems with Brann, but one fix at a time. I doubt this will be implemented since it would require more work than just changing a number, but I think it’s the best solution for fixing this interaction.