Knowing how egotistical these villains on the Equestria side of the mirror, they will take any opportunity to invade the Equestria girls world if it presents itself. They will underestimate humanity. They will see them as being defenseless and weak as they have no magic to defend themselves from the invasion. They will go down there and raise heck for the humans.

I'm pretty sure that King Sombra in one of the alternate timelines in "The Cutie Re-Mark" likely has the mirror in his possession and was probably planning to or considering invading the human world. He'd have to also build larger portals for his armies to march through and eliminate the magic problem that the mirror has.

I'm pretty sure other villains in the other timelines "The Cutie Re-mark's" have the mirror in their possession and had the same ideas and ambitions on conquering the humans. Nightmare moon, who much like she did when she saw the cutie map, would have also taken the chance of invading the Human world and bringing about eternal night upon Humanity. Like Sombra, larger portals would be constructed as well.