1. Vulcan was secretly kept as a prisoner by Black Bolt and a big bad villain (probably a human that hates both the mutant and inhuman) found out and leak this information to the public.
2. Inverted Havok hears about this and go to New Attilan with his team to get the Inhumans to release his brother but they denied him because Vulcan is their enemy and since he is way too dangerous to be released. Havok lost his patience and attacked but accidentally kill a royal member in the process thus triggering a war between the mutants and inhumans.
3. Both side will launch a massive attack at each other and they will keep fighting up until to the final issue of Inhumans vs. X-Men where the big bad villain appear to be holding Medusa's head like he's the next Perseus and told them that they have all been played like a fool. Cyclops suddenly appear and optic blast this villain guy to death. Crystal will also appear along with Cyclops and announce that Cyclops will be king of the mutants and that she will be his queen as a sign of diplomatic relation between the two races. Crystal will announce that she will help build a New Utopia with Cyclops for mutants to live there while the inhumans will live in New Attilan.
P.S. See how an important Inhuman like Crystal is not in the cover below? Yes, this theory is definitely going to happen...