The rod would be interesting and I nice nod to norse mythos. How exactly would the rainbow bridge look? Is it more like a rainbow stair?
So I see three options for implementing the Rod of the Bifrist:
1. The rod comes in dungeons, easiest to implement but makes getting to the Frist completely luck.
2. If we wanted a mob to access to the Frist, I think adding a whole new one just to get their is a bit too much. How about Ice Trolls (or Yetis, I prefer that name) that rarely spawn in the Overworld cold biomes? When defeated the Overworld Trolls drop the rod.
3. Or we could move the ice castles to the Overworld and have the Baron serve as a miniboss guarding the entrance. I imagine the baron looking somewhat like the ice king from adventure time, with the villager model.
Of those three, #2 looks like our most sound option. Because #1 leaves there to be a limited number of those in the world, and despite the sheer size of MC's world, is not multiplayer friendly. #3 just doesn't quite feel right. I mean, a miniboss to enter a dimension that's non-linear feels... Off. Even if the dimension was linear progression.