So just for creatures, but it might work for the other types of cards, I broke down the tiers of creatures by mana cost. There are 3, 5, 8, 12, 16, 20, and 24. 3 mana creatures are fragile, there are not a lot of them but they exist. If u sneeze on a 3 mana creature it dies. One exception is sanguine hunter, he special. Next is 5 mana, which is really 4-6 mana creature, these are basic creatures, build a swarm with these. 8 is 7-9, these creatures can be strong but can also get killed relatively easy. Timber wolf is thought to be the best example of this group but really he is the gold standard, he is top tier for this group. 12, 11-13 the strengths and weaknesses of this group can be comparable to group 8. The difference with group 12 is they have special traits, abilities, attacks, or effects which set them apart. 16, 15-17 they have added strengths from previous group. These are your heavy hitters enough said. 20, 19-21 these are your champions, chances are u have only one card from here in your deck. It is the face of your deck u might say. It will wreck havoc on your opponent and his creatures, as we'll as make them divert actions and resources to dealing with it. This is your big gun. Group 24 is God tier, they are crazy strong and ridiculous to kill but not impossible to destroy however the best way to deal with this creature might be to not deal with it and just go for the enemy mage.
I think looking at creatures by what mana tier group they belong to is important when deciding/understanding how you will build your book and your mage summon creatures. Because this is a customizable game u can have any creature u want in your deck regardless of school, but having in school creatures is always an optimal strategy for SP efficiency and summoning potential. Look at your mage's in school creatures and figure out how many different creatures and which tier they reside. There is an importance and difference in amount and variety. The Siren currently(not including 2 newest expansions at Gencon) has only 1 creature in the 5 mana tier and has 6 cards of that creature. So her amount is 6 and her variety is 1. The wizard has a variety of 2 at the 5 mana tier. While beastmaster has 8 variety and thats just counting animals. Going back to the Siren, if we look at her 8 mana tier and her 12 mana tier, she has 2 and 6 respectively. We can think of 5 mana as a basic creature, an 8 mana as a warrior, a 12 mana as a specialist. With 6 varieties of 12 mana creatures we can say the Siren has an easier time building or utilizing creatures with specialized skills and traits than swarm building. Beastmaster biggest strength is versatility when it comes to creatures and building. He can go many different directions because he has a variety of creatures in each mana tier. Beastmaster have all these pieces to a puzzle and their challenge is deciding what the imagine looks like.
This was just my observation and and thoughts on creatures, mana cost, deck building, and utility.
