The idea stuck me right before going to sleep while discussing that unlocking more weapons and items thin the probability to spawn ... like a deck building. Then the ideas kept coming and I had an hard time trying to sleep (Proving that creativity is a curse). The concept is pretty straight forward and it implements most of the video game's mechanics via deck building. Here are the core mechanics ideas:
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There is 3 deck, the monster deck, and the player deck and the market deck.
You flip a certain number of cards from the monster deck each turn. You must defeat them with the cards in your hand. Defeated monsters goes into your discard pile, undefeated monsters make you lose HP.
A "Stage" will stack cards in different ways. On the first trier, there might be like 10 cards, shuffled. You flip 2 cards per turn. Once the deck is depleted, a new stack of stronger monster is created and now you will flip 3 cards per turn, then 4 , etc until the end of the stage or until you die.
Your character will determine which starting cards you have in your deck. Like any other deck building game, cards you purchase goes into your discard pile.
There are 2 type of cards you can purchase: Weapons, allow attacking enemies, Support items, give special ability for the duration of the turn, or maybe longer.
There will be a market of 3-5 cards to purchase like in Star Realms
You can discard monster cards from your hand as EXP to purchase new items in the market.
You could discard a matching pair of card (weapon + item)to get access to more powerful special weapons.
If you defeat a mini-boss, you get a chest and flip 1-3 market cards you get for free.
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I am not sure if that idea could be rethemed into something else. It has some similarities to dungeon delver. I intended to make a solo game, but a coop game is another possibility.
I won't get deep in the sleep issue, I read a lot on the subject and I work in a psychiatric hospital. I am using biphasic sleep when I don need to wake up in the morning. It seems like the only solution that works. But I would need to eventually work on night shift to be compatible with my work.
Vampire Survivor it actually a indie video game inspired by Castlevania. It's basically a game where you walk around in a stage swarming with monsters and your character auto attacks (no buttons required). You collect XP, gain level select new weapons or items.
As time progress, the monsters becomes more powerful, but you can also become very powerful if you gear up upgrades correctly and kill enough monster in time. Else, your level will en up behind and you'll have problems surviving.
So the deck building mechanism seems to simulate well that snowballing progression.
A re-theme idea I had could be that you are a battlemage building up power to destroy swarms of monsters.