Basically replace the members of the Star family with a Nicktoon character. My idea was:
Jimmy Neutron replaces Squidina in Rock Bottom.
Timmy Turner replaces Cecil in Ripped Pants.
Danny Phantom replaces Bunny in Neptune's Spatula
Invader Zim replaces GrandPat in Help Wanted
Lincoln Loud replaces Granny Tentacles
(Ignore the fact none of them can breathe underwater naturally.)
Happy 22nd anniversary to The Adventures of Jimmy Neutron: Boy Genius!!! This was one of my favorite TV shows I grew up watching back in the 2000s since I was 6 years old.
https://mobile.twitter.com/NickBrawlGame/status/1522599567966892032
They use the line "Back to the lab again" which strongly suggests it will be Jimmy Neutron! Garfield and Shredder were also teased in similar ways. Jimmy also has unused announcer lines in the files along with Cindy Vortex and Rocko and Filburt from Rocko's Modern Life (and many unused SpongeBob, TMNT, Avatar, Rugrats, Danny Phantom, and Wild Thornberries characters). Then Timmy Turner will be the only member of the original 4 Nicktoons in Nicktoons Unite to be absent (they teased Timmy back in December though by using the words "Fairly Odd" to describe stuff they are working on). Who are you hoping to see join?