Board Game Arena

I often played this game before, and I think it's better to use swap rules(like the hex) or ban rules for the first step to avoid huge first-hand advantage.

The pie rule (one player starts, the other chooses whether to stick with that or not) is a nice way to add thoughtfulness to any 2p game with firsst-pplayer advantage.

I have another suggestion, less technical and more "Graphical".
When one mini-board is won, I would remove the highlight of the winning line and "fade" the pieces, covering it with a big X or O.
A bit like this image https://commons.wikimedia.org/wiki/File ... ictory.png

Keeping all the pieces and the highlight creates a bit of "information overload"