The main character would have bright green hair and Franziska's sense of fashion.
The rival would looked like a young Manfred von Karma and had a pointy Beard of Evil.
The mentor would be a big, hulking man.
The assistant looked more like Ema.
The detective would be a hot woman.
In-universe example: At the end of 1-4, Phoenix thinks he would've been a prosecutor because Larry framed him for stealing lunch money and Edgeworth thinks he would've been a defense attorney if his actual father wasn't killed by the man who would become his adoptive father and mentor. There's a fan fic based on this premise.
An early concept for Wendy Oldbag gave her a much younger Brawn Hilda look.
This blog post from one of the members of the localization team gives a few for the localization, some being very... interesting.
Among the English names considered for Diego Armado were Joseph Cuppa, Xavier Barstucks, and William Havamug, really
Luke Atmey's original catchphrase before "Zvarri" was "Schwing! Additionally, Godot was going to drink bourbon whiskey and smoke, but the production team worried it'd have a bad effect on kids.
There were several different fonts considered for the "P" on Feenie's pink sweater
Because there wasn't much figured out about him ahead of time, Gumshoe underwent an evolution in character. For example, he originally was planned to be into horse racing
Maya originally being planned to be a lawyer-in-training under Phoenix. In a different blog post, it was mentioned that Phoenix met her and Mia under very different circumstances, that Turnabout Sisters would have been the first trial of the game (it was bumped to second as it was felt it was too big to be a lead-in), and Phoenix originally would have been a detective who was arrested for murder after he walked into his office to find a corpse. He then would have taken up his own defense because of how incompetent his own defense attorney was.
Larry Butz went through a good many changes in character, originally being a 9-to-5 office worker and an Average Joe and ultimately going through some editing when the first trial was unexpectedly added to the game and included him. At one point, he was a tough guy who went around shouting how he wanted to kill everyone. This ended up being changed to him shouting that he wanted to die.
The Psychlock concept was originally envisioned as a single large glass lock that had to be broken through. This had to be scrapped as too difficult to pull off for the Gameboy Advance.
Back when Phoenix would have been a detective, the game title was "Surviban: Attorney Detective Naruhodo-kun" ("Surviban" being a play on words between "survival" and and "saiban"). Before that, planned titles were “Attorney Brothers Three”, “Boogie-Woogie Innocence”, “Bingo Bengo” (another play on words, suggesting hitting the nail on the head "bingo!" in court), “The Beautiful Verdict”, “Amber-Colored Testimonies”, “The Objections in Your Eyes”, and “Let’s Raise Hamsters” (one of the people on the production team wanted the lead to be a hamster).
The opening of the first case of Justice For All was going to have a giant devil looming over Phoenix, instead of the Judge (inspired by the opening of Devil May Cry)
Justice For All was planned to have five cases, but the last had to be scrapped because of lack of memory. It was later reused as the third case in Trials and Tribulations. The third game also had many things cut for memory reasons, including a design for a much-younger Gumshoe in the first trial and Oldbag showing up (there was just enough room for a cameo in the credits, and they were still unable to put her in a lei, which they wanted to do to suggest she'd been in Hawaii).