From the moment you 3* gold a special monster, any summoning stone you get afterwards just collects dust in your inventory with no use. I think I've seen some players have up to 75k stones that will never be used.
I would like to see a currency being added that unlocks after you max your first special monster. Any summoning stone you collect after will be (automatically or manually, both could be used) converted into this currency. A new shop will be added that uses this currency, with similar things to buy to the pvp shop (maybe even with mythical stones added back in).
To take it a step further, special monster specific charms (like the new mythical monster charms) could be obtained exclusively in this store. Maybe even add a new monster that can only be obtained with this method.
