You can make huge fundamental changes like "you cannot connect 3 stones in a row" or you cannot make a certain shape with your stones. There are 'bad' shapes in go that you learn when you're young that almost never surface in higher levels of play (of course you would make a non-bad shape illegal). You can say every 15 turns you get 2 moves in a row and they are stagged (this is game-breaking). You can ban certain josekis ('standard moves' early in the game). You can make each captured stone worth 2 points instead of 1.

Every move, you flip 6 coins. If all 6 come up as heads, you swap seats and colors with your opponent.

This means that an advantageous board position is much less important, and it will be in both players favour to keep the board position very near evenly matched.

Different versions of Go have different rules for scoring.

Switch to a 21x21 board? Maybe consider a different board shape? Suppose there is a hole somewhere.