Merely allowing monstrosity to work in conjunction with the rest of the game would have been a good roundabout way to add the morpher class into the game. All those instincts work well as equipment already so don't really need additional things(until people start getting demanding).
The crazy thing is that they used some of the lore elements of Monstrosity in Adoulin. The same floating head guys appear in the Vagary cutscenes (as well as their master), and you need certain Monstrosity KIs to activate an Adoulin quest that explains who Lorissa is (yet they missed an opportunity to have a quest named Lorissa Explains It All).
Anyway, I wouldn't be surprised if they added something like Sinister Reign (which is getting more content somehow) for other expansions, where you could acquire Iroha's outfit similarly to how you can get August's armor and Teodor's coat.