No mainline games, no Prime games, not even remakes or reboots of older games. A completely new Metroid spin-off. It could have the same gameplay with different characters and location or we could see Samus in a whole different game. Pinball demonstrates that the possibilities are endless.
So, which is your wildest Metroid dream?
For example I would love a Dachora and Etecoons metroidvania. The main mechanic would be the collaboration between the different animals: you could instantly switch between them in order to solve puzzle and find secrets.
The focus would be exploration with little to no combat and the mood would be a lot lighter that the usual metroid, maybe even cute Nintendo style. A big inspiration for atmosphere and setting could be Yoku’s island express.
Lastly it could fill the gaps in the story, like how they got to the BSL station after Zebes or where they went after the events of Fusion.
I know this is the most impossible thing ever, but man, how fun it would be.