Mechanics for interactivity in card games
Magic's popularity is a bit of a curse because it feels a lot like any reasonable way of letting players act during the other's turn is a rip-off of it.

This isn't out-of-sequence play but it does allow counterplay: have certain cards take a turn to carry out their effect (kind of like Magic's suspend ability). I "cast" a spell, then pass the turn. On my opponent's turn, they can counter my casted spell or do whatever they want, then pass back to me. On my turn, the casted spell now takes effect if it hasn't been hit.

More powerful spells can take longer to come out, you can have spells that come out immediately (like most counterspells wouldn't have to wait a turn to work), you could have quicken or slow type effects that add to the turn-counter for spells, there's a decent amount of untouched design space there.