9. Super Mario Sunshine
Development Begun: late '90s
Although Super Mario Sunshine, in its final form, wasn't unveiled until a year before its release, it was really just another iteration in Nintendo's continued attempts at creating a sequel for the beloved Super Mario 64. Nowadays we're used to only getting one or two core Mario games per console generation, but back then there was still hope Super Mario 64 2 would hit the N64, long before the GameCube was ever released.
Mario Creator Shigeru Miyamoto said in 1997 that Mario 64 2 was being developed for the (terrible, terrible) Nintendo 64DD, a failed peripheral for the N64, and he confirmed in 2006 to IGN that elements of that game went into future Mario titles.
The same goes for Mario 128, a GameCube tech demo that eventually influenced everything from Sunshine and Super Mario Galaxy to Pikmin and Metroid Prime. Despite certain criticisms, Sunshine is a solid title in Mario's legacy, so all that experimentation seems to have paid off.