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I would think some medium between the Exp. Share of Gen 6-7, the Exp. Share of Gen 2-5 and the Exp. All of Gen 1 could be achieved. If you had it that the Exp. Share was an item that could be given to multiple Pokemon simultaneously, that way players of both Exp. All and Gen 6+ Exp. Share get that aspect of the Exp. Share, but to ensure it isn't broken like the Gen 6+ Exp. Share you have it divide Exp. like how Gens 2-5 did, ensuring that the more Pokemon that have the Exp. Share equipped the less Exp. each gains. You can effectively modulate difficulty that way.