There's a huge number of things you can do within Rainbow Six Siege, and even more are being added all the time, with Operation Ember Rise adding a deployable shield with an explosive charge, and a grappling hook, for example.
You'll be surprised to learn therefore that there's actually been several ideas left on the cutting room floor, some of which a pretty wild, including a canine unit and even a dual-wielding operator that would have required players to play the game in an "entirely new way".
While both ideas are on the cutting room floor and are unlikely to ever see the light of day, they were recently discussed in an interview with Wired, where presentation director Alexander Karpazis discussed the ideas.
In the end, the team had to ask themselves if players would be "okay" with shooting a dog. It's a good question, most games feature petting dogs as a feature rather than killing them, and it seems like one thing that the team decided was a step too far.
The dual-wielding concept was one that made it to the concept stage but never got to the prototype stage, as such the developers are unsure on the impact it would have on the game. That being said, never say never, but Karpazis stated that the team "don't know if we'll ever get there in the near future."