Stop N Swop
A special feature in Banjo-Kazooie in which you would collect six colored eggs and an Ice Key and then transfer the data to Banjo-Tooie using a method called "hot swapping" or taking out Banjo-Kazooie in the middle of gameplay and putting in Banjo-Tooie.
It would have been awesome. But it couldn't be done, apparently. Due to upgrades to N64 hardware around the time BK came out, the time you get to swop games would go from 10 to 1 second. The supposed features you would have gotten in Banjo-Tooie were....
Jinjo in Multiplayer Mode
Breegull Bash
Dragon Kazooie
Homing Eggs (AWESOME)
Extra Bubbles
Devil Bottles
There have been craploads of theories and such. One which I thought was the most interesting was the connection of Blackeye the Pirate (only seen in pictures in Banjo-Kazooie, but is a character you can talk to in Banjo-Tooie) and Stop N Swop. There were hints about him apparently trying to hunt down all of the Stop N Swop eggs. He had his own room in Mad Monster Mansion (where two Stop N Swop eggs reside) In his cabin in Rusty Bucket Bay, a SNS egg can be found on his bed. In another room in the same level, there is a map with an X on it. The layout on the map looked similar to Treasure Trove Cove, and the X was Sharkfood Island, (which by putting in a code you can raise up from the ocean and find a SNS egg inside) In Banjo-Tooie, you can find Blackeye in his room in Jolly's Bar in Jolly Roger's Lagoon. You can see pictures of something familiar in his room. One is of Treasure Trove Cove, one of his Sharkfood Island, and the other is of inside Sharkfood Islans, where you can also see the Pink SNS egg. So there are people that think that Blackeye would allow you to perform Stop N Swop and get the items from Banjo-Kazooie to Banjo-Tooie. I'll explain more about this later on