Here is one item I’m a little surprised no one has done yet, even as a home-brew project: a bed based on a WWF/WWE wrestling (or boxing) ring. It would feature the ropes and turnbuckles of a ring, and the wrestling mat graphics would be printed on the duvet and/or sheets.
If this was made, the temptation to recreate wrestling acrobatics would be huge. I would make the corner poles telescoping (would descend when there is weight on them). The bed would need to be sturdy in case the kids like to jump around. As a safety precaution the ropes could have a break-away feature (like your Xbox 360 wired controller).
This design is more for kids, but if adult wrestling fans want one, be prepared for non-stop ribbing.
BONUS IDEA: A baby crib shaped like an MMA Octagon. That would be hilarious, seeing a toddler crawling around a blood-stained UFC ring.