As far as I know there's really only one video trailer for he cancelled Kirby game that was shown of at one e3 AGES ago, never to be mentioned again.
The only trailer known
the Kirby Wikia page has a bunch of info, some trivia, and quite a few unique screenshots
http://kirby.wikia.com/wiki/Kirby_GCN
examples - more in the above link
Knuckle Joe,
Scarfy
Waddle Dee
Parasol Kirby,
Golem
[more screencaps in the above link]
This articles with the developers from RTDL might give insight to evolution of Kirby game cube to RTDL. Screen shot of other cancelled attempts and information from the developers
http://iwataasks.nintendo.com/interviews/#/wii/kirby-dream-land/0/0
Iwata
During that time, screen shots were shown and release dates went unannounced for a long time. Then the Nintendo GameCube system changed to the Wii console. Miyamoto-san says that video games are something you never really complete. It's hard when a game simply refuses to come together.
Kawase
We wanted to bring it out when the fans wanted, but the movement wouldn't feel right or we wanted to deliver more of a surprise to the fans, and we couldn't bring one out in final form. We experienced that difficulty for 11 years.
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Here's another articles about Kirby Game Cube. Though it basically links to the same stuff and information I have. The same screens shots, the interviews from everyone behind RtDL, and one e3 trailer that exists
http://www.unseen64.net/2008/04/09/kirby-adventure-gamecube-unreleased/
As far as I know that's all the info out there. There are no other videos of the game since it's e3 trailer, since development and release date remained under wraps since it's reveal. It remained in the darkness up until Epic Yarn (11 YEARS GUYS OMG I WAS DEAD AND DYING) was revealed as the next Kirby game.... than finally RTDL trailer.