I know shard (as in gear shards) converters have been suggested before, and likely PG doesn’t want to introduce new buildings, or mechanics to cope with this, or the possibility of it unbalancing something so:
We have a trading post each season, hosted by the resident Reginald, so we could have another mini event alongside (maybe crystal caves to fit it?) a PvP, or otherwise, based on a shard trader, that is all about trading, or converting shards - whatever way or lore connections that PG wishes to make.
This doesn’t then require any huge new mechanics, or designs, and they could steal the Reginald idea by just making a different rider host it.
It could be a sorcerer, like Antonius, using their sorcery to transform shards into another type (so the ‘tax’ comes from conversion rates due to casting the spell), or the pirate rider Swann requiring rubies, or gold to make a shard trade, etc etc etc. I’m sure they could figure out if they needed conversion rates and payments etc to make it happen how they please.
Being a once-a-season event might also prevent it from being too abused, and give PG control on how much it can be used and in what time window, in case they feel balance could be thrown out.
Hell, if they wanted to, they could event add it to the trading post, as a trading market, with different ‘sellers’ being different riders - so a stall doing something with black pearls maybe? Or a stall that has hefty taxes to convert fire/ice shards into electrum?
It could allow them to slowly move currencies to new ones, and allow reduction of redundant ones without removing them from the game. A proper PG-led little market.
It was just a thought.