As for Monsters merch, my outside-the-box pitch is for some kind of "Build a Laboratory" concept, taking inspiration from stuff like Creepy Crawlers and the Doctor Dreadful lines. Individual units with specific gimmicks that you buy separately, but which can be linked/joined together so kids can construct their own mad science labs! With Monsters-specific branding, of course.
And for older kids/adult collectors, a line of Universal-exclusive, highly-articulated Monster action figures (like NECA or McFarlane Toys have produced) would be nice.