If you want to make this more balanced, I would suggest removing the ability to overlap any Rank 4 on its own, so that a "Rank-Up" Spell card is necessary.
It also has no retreat cost, meaning if things are getting hairy, Starmie can eject itself from the field without needing to get set back up with Energy
For our Commander players, we created 6 new rares and 2 new mythic rares that are intended for Commander play
Heres a list of everything you can hope to open in this slot: Cavern Whisperer $0.99 Cloudpiercer $0.99 Dreamtail Heron $0.99 Migratory Greathorn $0.99 Vulpikeet $0.99 Archipelagore $0.99 Auspicious Starrix $0.99 Boneyard Lurker $0.99 Chittering Harvester $0.99 Huntmaster Liger $0.99 Insatiable Hemophage $0.99 Majestic Auricorn $0.99 Porcuparrot $0.99 Regal Leosaur $0.99 Trumpeting Gnarr $0.99 Glowstone Recluse $1.49 Necropanther $1.49 Lore Drakkis $1.99 Parcelbeast $1.99 Yeah, theres not much more to say about these cards, unfortunately
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In most cases, the controller of the attacking creature will still become the monarch as it is likely their turn