Ok but well ... you based your hypothesis on the excistence of Star Fox Grand Prix and as seen lately, this game got no showing despite the heavy rumors it got a some point.
The origins of these rumors go back to a post made by an "insider" on SF's reddit which was soon followed by outlets looking it up thinking it might make a good click-article. Vergeben maintained the existence of this game and yet when it wasn't shown during the March Direct, he vanished.
Oh yeah, the explanation linking the racing feature in Starlink and the supposed SF Grand Prix is bogus due to timeframe problems.
Now, what did Retro do in the meanwhile ? I think they had some problems with their staff and management following the 2014 release of DK TF. Despite that, they ported DK TF on Switch and also worked on the port of Metroid Prime Trilogy HD but it just wasn't announced yet cuz of the delay for Prime 4.
Whatelse could they have worked on ? An original IP that had probably Sci-Fy themes in it's conceptualisation, sadly they never got far enough with it to actually start developping the title. Some of it's design might have been reused for the eventual demo they showed to Nintendo to convince them to take the MP4 project into their hands !
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