> Is anyone interested in discussing this?
Ooh yes please lets dig in! I haven't played Eternal myself, but I have played MTG as well as Hearthstone and a few other deck building games. It's interesting that Eternal is a 75 minimum card deck, that seems enormous after playing hearthstone (which required exactly 30 cards, no more no less, and no specific composition).
Something that might be helpful first is to classify which types of deck-building card games we are talking about. There are a few main types that come to mind for me:
Mana-Curve Games: This would include MTG, Eternal, and Hearthstone. Games where you have some ascending set of resources that allow you to play more powerful cards or more less powerful cards as the game progresses. You also start with a static set of cards.
Static Resource Games: Tabletop games like Dominion (I can't think of any others ATM). These are games where your resources each turn are defined by something static, like the ability to draw 1 card, and play 1 card (with the potential to gain the ability to play more cards with a given card). I mention these because they are often simpler and may be easier to start analyzing.
Asymmetric Games: Games like Android: Netrunner where there are all kinds of wonky things going on depending on which side you are on. Similar to Warhammer or something like that. We probably don't want to talk about these.
That being said, it sounds like you'd like to discuss Mana-Curve Games. I think there are probably a few terms we might want to consider:
Card Value: This is difficult to define for Mana-Curve games, especially with synergy. Taking synergy out of the equation, you could simplify down to something like card power / mana cost. Adding synergy back in, this may actually be a function of the other cards in the deck.
Deck Size: Number of cards in the deck.
Average Card Value: Just Card Value / Deck Size.
Draw Potential: Based on the cards I have left in the deck, what is the card value I am likely to draw. This would be Average Card Value of the remaining deck.
Draw Depth: What percentage of the deck am I likely to go through over the course of the game. You mentioned that Eternal is often about 30% of a deck. For Hearthstone it is probably upwards of 60%.
Key Cards: These are the cards that the deck is built around. In Hearthstone where the Draw Depth is high, this can often be as few as 2 or 3 cards with everything built up around them.
I think the steps you listed above are explaining how adding an additional card of lower Card Value reduces the Average Card Value of your deck, therefore reducing your Draw Potential over the course of the game.
I need to run now, have more thoughts to give later.