Velvet Dark

Velvet Dark is a canceled project that was planned as a sequel or spin-off of Perfect Dark.
The game was designed as a third-person stealth action game following Velvet Dark, Joanna Dark's sister though the relation wasn't stated in Perfect Dark.
Velvet appears as the co-op character in Perfect Dark.

Likely beginning as a Nintendo 64 title, the game was canceled at some point after the GameCube was announced in 2000.
The table of contents page of a design document for the game, dated the 30th of October 2000, was revealed on twitter by Gregg Mayles in 2015.

Compatibility with the Game Boy Advance was planned, probably similar to the way content in Perfect Dark could be unlocked with a connection to the Game Boy Color game, also titled Perfect Dark.
This would also mean there were plans for a Game Boy Advance companion game of some kind, though no work on a connected title has been revealed.