Mirrorpool can also copy instants, sorceries and creatures we control by sacrificing and paying some mana, which is also great in Eldrazi Tribal where he have many high cost, high reward cards
Its powerful, cheap, and brings far more value than it should
Then most of your creatures are tapping naturally
The 5-cost Tashigi is a massive mid-game Character that cant be removed by an effect thanks to her ability to rest a card instead and remain in play
It will also be focused on designing cards that could likely be printed on cards by Wizards of the Coast, and most likely in a standard set (planeswalker deck planeswalkers are balanced completely differently, and supplementary set ones are also separate from that) but card creation is still a creative act and, in the end, youre the only judge of what you can or cannot do on a card