Well. You can already spend your gems to get shards.

and anything along the lines of exchanging mystery shards for any other shard is completely unlikely to happen. This would make it too easy to get the champions you want, and it doesn't take much time to accumulate large quantities of mystery shards as it is, atleast for many of us, for example I have over 1500 right now.  They would probably have to change the shard drop rates in every content in the game, along with an inevitable change in the value of ancient, void and sacred shards would occur which would not be a good thing.

But, the idea of a system for exchanging high amounts of Mystery  shards for, say, Silver or potions might be a good idea.

Also, consider that the mystery shard drop and availability is proportional to the other shard drop and availability, along with the % chance to pull epics and legos from these shard types.

That is very understandable. Not sure if there are rules against necro'ing threads but here I am lol.

So converting shards into better shards would be a no-no, but what about this new idea, which perhaps more accurately represents the idea of "upgrading shards."

You can sacrifice shards to increase the rarity percentages for INDIVIDUAL shards. For example, sacrifice 100 Mystery Shards to upgrade a Mystery Shard, increasing the chances of a rare champ. Same for Ancient, allowing you to increase the chances of Epic and Legendary champs, so on so forth. That way there could potentially be more bad luck protection were you to "fully upgrade" a shard, but the trade off is you are throwing away many equal quality shards with regular drop chances.

Even if they did what you mention though, allowing the trading of shards for currencies, I think would be pretty cool.

Hey there! As appealing as your idea sounds, I'm afraid it's not something we're likely to implement. There are still plenty of ways to get Shards and improve your chances with their quantity.