This Ask Reddit thread by one of the show’s writing assistants details a number of plot points that were seeded but never developed before the show was cancelled. Pardon my liberal block quoting.
The family cabin:
We were all devastated about the cabin on the truck. Especially Mitch. There was so much planned for that. The cabin was going to be the new driving gag à la the stair car. Multiple characters (Johnny Bark, Tobias (after Lindsay kicks him out) and Oscar) were going live in it simultaneously, completely oblivious to the others also living there. They’d never actually encounter each other because they’d never be there at the same time. So each thinks he lives alone. Plus the cabin would frequently be on the move, relocating and turning up in various places: outside the model home, the Balboa Towers and Bluth Company parking lots, impound lot (after Lucille has it towed), Oscar’s lemon grove. Oscar was going to be the only one living there who would always find himself in a different location that wasn’t the same as when he entered.
One of my favorite setups that we never got around to doing was the Sally Sitwell doll. It’s first mentioned in “Out on a Limb” and “Hand to God” when Maggie Lizer (ass off) jokes that she thinks she had one of those dolls as a child. It seems like a throwaway gag but there is a Sally Sitwell doll that was modeled after the real Sally Sitwell. That hairless freak Stan Sitwell stole the idea from George Sr. who was going to model a doll after Lindsay. This was going to be the another revelation of the back and forth stealing (Sticking it to the other guy) between G. Bluth and S. Sitwell that has gone on for years that started with the Bluths adopting Lindsay’s out from under Sitwell. There are other hints to this throughout the show, like when Sitwell hires/steals Gob away from The Bluth Company to work for him and play on his softball team.
Tobias is black:
One of Tobias’ real parents were gone to be black.
The cover of “The Man Inside Me” magazine features a black man. Elsewhere Lindsay says, “People hear the name Tobias, they think–big black guy.” Tobias: “Well, obviously, I’m not a big guy.”
And some other things that were in the plan. Season four:
For the most part it would have picked up right where season three left off. Many of the pieces put into place at the end of season three would have continued over. The main family arc was going to put Lucille in a minimum security rich white people prison where she becomes something of a mother figure (with a little Martha Stewart throw in there) to the other inmates. This of course would have offended her children, whom she never mothered. George and Lucille Austero start up a romance with Lucille 2 assuming the role of matriarch of the family with most of the kids actually taking to her, except for Buster, with Gob being the most mother-starved of the group.
I can tell you this about “S.O.B.s” though: every celebrity who appeared in that episode was to appear in later season 3 eps. John Larroquette was going to be “dating” and then later stalked/kidnapped by Kitty.
Had the show gone on longer, Buster was likely going to lose another apendage. All those robot references were foreshadowing Buster becoming half-machine.
Lots of good stuff in the thread.