This game is called Vortex Wars, so I think you should actually have something in the game called vortexes. This would also help to add a new feature to differentiate the game from online risk.
You would get 1 vortex every 4 turns. You would be able to place a vortex only on your land, and only you can use your vortexes. If someone else conquers that land then you can directly attack it with your reinforcements at the end of your turn, using as many as it takes to beat it, teleport troops through another vortex to attack it, or implode the vortex, causing it to take a small bit of troops out of that spot, while destroying the vortex.
Kind've a complicated idea, but I would like it.