Metroid Prime is basically akin to Mario Kart.
Sure it's a spinoff but it's a huge release with high budget.
Ridiculous nonsense. It's not
anything like Mario Kart, a cute spin-off that makes absolutely no sense as 'cannon', it's bad guys and good guys clowning around together in another genre. And Mario is already a dream-ish lighthearted no-timeline series.
Metroid Prime is same genre, and has all the same principal characteristics (Samus hero defeating bad space pirates who want to use a bad bioweapon), plots, setting, characters, enemies and tone.
Blast Ball, Pinball, Prime 2 multiplayer, these are the Mario Kart style spin offs. Federation Force is the closest thing to a 'main' spin off, as you do not play as Samus.