I'm not sure that I really care to see their "adult relationship", it would seem too dissimilar. Of course, they could always add some mecha and call it a shounen show.
I know there's lots more Grimm tales to work with. The ones that they already incorporated in this show would be hard to stretch farther is what I was thinking. Again, tends to end up too dissimilar. That's the trouble with saving the universe, you're not too likely to have to come back for a repeat.