The rule could be anything, as funny or as serious as you want. The universe will progress in a similar way that it has up until this point, unless your changed rule prevented it from doing so.
Some examples might be:
- The invention of currency is not allowed.
- Iron is slightly less stable.
- The Ancient Greeks are able to cultivate Silphium, which does not go extinct now.
This is a problem I’ve always had with Square grids in D&D and it never occured to me that from character perspective a character is warping space to move slightly further for the same amount of movement.
Also non Euclidian! Hexagons (the bestagons) also tesselate and fix that problem nicely