By leveraging different sized tokens, they can keep training each other, growing every time they attack
Would you build it truly five-color, or would you restrict the colors or even go mono-color like Ive proposed here
If they were to only offer two sets for each generation of Pokmon (which they likely wont, if you understand how the physical TCG works), thats 18 months worth of content on its own
A 3/3 Dragon with flying and haste for five mana, whenever one or more Dragons under your control deals combat damage to a player, you can goad a creature that the player controls, forcing that creature to attack one of your other opponent's next turn if able
This gives you all sorts of versatility with your lands, where instead of just waiting until you need them to tap mana, you can tap lands you don't need or might not need to commit mana to and use them to make a bunch of Treasure tokens