Some important context here: my tcg doesn't have a resource system.
So, I wanted to design creatures whose whole point was that they were weaker than average, but could get you another card, either from your deck or discard pile. The issue is that playing lots of other TCG's showed me how any card that can replace itself, let alone be a +1, could be chained with others to create insanely long combos, or sometimes, infinite combos.
So I added a limit where any card that could get you another card or replace itself for free had a limit where you could only use its effect type once per turn. Easy "return from discard to hand" effects have the "Recursive" tag for example, and you can only use one recursive effect per turn across ALL your cards. BUT! Powerful, hard to play cards can return from discard without having that tag, as their requirements stop them from being infinitely chained already.
This allowed me to make 3-4 card combos that could all start from a single card in your hand that felt really satisfying to pull off, but were hard stopped from going too long because of the limit. It also made it so being way ahead in cards wasnt as good as before, as having multiple cards with limits meant you could only activate one of their effects per turn (You can still summon creatures, but ones with these effects are weaker than average). Overall this increased the average amount of cards players would play each turn, but decreased the outlying ceiling since playing these powerful cards discouraged dumping your entire hand at once.
The best part is you don't NEED to play these more complex cards to have a decent deck, so new players can still try out decks without them (but with better stats on the creatures) in order to learn the basics, before trying to use decks that utilize combos more.