Rerouting ley lines to the Nexus so he can yeet all magic into space.
Basically a planet sized flare gun alerting anyone and anything that could potentially come and harm Azeroth.
His plan would've left Azeroth completely defenseless and who knows what that would've done to all the life on Azeroth, especially all the species and areas that are directly dependant on it.
Yogg was almost free at the time and we already knew the Legion was gonna return at some point, so either of those forces would've just taken over the planet.