I swear to god I'm not making this up or shitposting.
https://en.wikipedia.org/wiki/Ultimate_tic-tac-toe

There's a tic-tac-toe variant where you take a 3x3 of tic-tac-toe boards and it's an actual game instead of just trivial to solve and always ending in draws.
Starting in the middle as normal, the next player must then play in the board corresponding to the square selected in the middle.

For example, X places their mark in the top right of the center board, then O must play in the top right board.
If O then places their mark in the middle-left square of the top-right board, then X must play in the middle-left board. (If a player must place in a completed board, they may. If they must place in a full board, they may choose which board to place in.)

The player who wins three full boards in a row wins, or if none of them it's the player who won the most boards over-all.

I suggest allowing a 3x3 of tic-tac-toe boards to multiblock together into this structure.