I believe if transaction fees become an alternative to RCs, they should be for all type of transactions, including custom jsons and voting or posting. Not sure if it would be good or bad, apart from the work needed to make this happen, it would be great if some cons would be provided by someone who can think of them.

About conversion fees, for HIVE -> HBD there's already a 5% fee. I don't remember what happens with the fee.