英語版Wikipedia見たらスタッフが語ったStop N Swopの内容が普通に載ってたわ

https://en.wikipedia.org/wiki/Banjo-Kazooie_(series)
In an August 2004 interview with ClubJoe,[13] an anonymous ex-Rare employee explained in detail
how Stop 'N' Swop was going to work:

Only four eggs and the ice key were involved - two eggs (the cyan and yellow eggs, found in Mad Monster Mansion and Click Clock Wood)
were "bad eggs" that would not be pointed to by Tooie,
and would prevent Stop 'N' Swop from working since only hackers would have access to them.

Blackeye the pirate would give out sandcastle codes in Tooie in return for completing tasks,
allowing the collection of the "good eggs" and ice key in Banjo-Kazooie.
By going through the gold warp pot in the Grunty's Furnace Fun with the appropriate items,
the ice key would sparkle and open one of the locked doors in the transformation room seen
in the GAME OVER sequence (owing to a programming mistake, the ice key would disappear afterward,
so this would only work once).

Going through this door would result in Grunty complaining about Blackeye, and a message to swap cartridges.

In Tooie, the four eggs would be used to enable special transformations - Kazooie to a dragon (the only one remaining in the final game),
Banjo to a polar bear, and two unspecified transformations that were never coded owing to the abandonment of the feature -
as "fun and useful, but not needed" bonuses for those who had both games.


元レア社員のインタビュー全文
https://www.gamekult.com/forum/t/interview-dun-ex-employe-de-rareware/72170