A lot of balance and care goes into choosing the cost of a card, more than I can express here, but looking for similar effects on existing Magic cards can give you an idea of a fair costfor example, a ramp spell that puts two lands into play ought to cost 4 mana, per Explosive Vegetation
Each plane will get rated in three categories, receiving a score between 1-5 for each: Fun (does this make the game more fun to play?), Impact (how big of an immediate effect will going to this plane have) and Chaos (how does the chaos effect affect the game)
Unlike later MTG sets, these early prototypes used artwork sourced from comic books and pop culture, making them a fascinating piece of Magic game history
A spell cast using flashback will always be exiled afterward, whether it resolves, is countered, or leaves the stack in some other way
Any ways to increase counter production, like Doubling Season, would be welcome here
Each has fun new abilities that must be activated to reach full potential, changing up play to your advantage