I agree—making it easier allows more people to enjoy it.
Yeah, synergies are a super essential part of any deck-building game. For example: a "Super Rock" card that also works like a normal rock, but can destroy any other card. Maybe it becomes more powerful the more rocks you have in your hand—encouraging the player to collect as many rocks as possible.
Or a card that destroys your own cards, which sounds bad… but you could pair it with another card that gives you +10 coins when destroyed , also a card that doubles the money you earn in a round ( giving the player the option to farm coins during early rounds )
I could keep going, but it really depends on what kind of gameplay you want to build and what your vision is for the game .
Do you want to add other card types? Things that work completely differently from what you have now—like spells that don't destroy opponent cards in the traditional way? For example: a card that lets you draw two cards (even if your hand exceeds the max hand size), or a card that does the same thing to your opponent.
Do you want to add more mechanics to the battle system you already have?
also your game nature ( being a incremental game ) make it unique and very diffirent than other deck builders
Honestly, my ideas might not even fit your game. I think that's something you never really know until you test it out .
Hope that helps you somehow. I once tried making a deck-building top-down shooter myself… so I really dove deep into the genre and fell in love with it.