A conclusion following that G&K has a surprise for the Nadalind fans. Unexpected and a suprise but there will also be a sort of sad side to thier fairy tale.
First part (the happy moment) - Bonapart put a cursed ring on Adalind's finger and the curse will be lifted only if she married Renard, BUT what Bonapart did not specify was, it had to be specifically Renard, but in general. As long as she gets married, that curse will be lifted. We now know, Nick loves Adalind as do Adalind
Second part (the sad but satisfying moment) - because of everything happening with the other realm, the big-bad, the stick and children, Nick connected with the stick and end up connected to the other realm, Nick will need to protect both worlds, following his destiny for he has a Grimm destiny, to become the new Gate Keeper and leader of both realms, Nick's world and the Wesen world after defeating the big-bad (Renagade Grimm to be likely). He will ofcourse be watching over the Children and his wife Adalind and maybe sometimes, yes sometimes crossover to be with his family.
Nick knowing he has to leave his world to the other realm, he and Adalind decides to get married first to be together knowing they love each other and wanting to be together knowing that no matter what happens or how far apart the worlds they are, they will always want to be together.
This is what the conclusion will be like. A tearjerking/ tissue holding moment, a satisfying happy ending.