Better Mine Physics.

When a mine is placed on a portal floor, and said mine is already activated, it should still be able to fall through a portal placed under it.
To expand on this it would be pretty funny if the portal it's exiting is a launch portal, the mine should get launched as a player would!
In a lobby players might attempt to launch their mine across the map or onto objective. And it's counterable with skill because the mine can already be shot mid air, or destroyed via a well timed explosive.
A higher skilled player may portal their mine as another player is launching through the air and blast them in a new direction and kill most of their momentum.