Originally, Sonic Adventure 2 was going to only have 3 characters; Sonic, Knuckles, and Eggman. It would take the three styles of gameplay that worked in the original Sonic Adventure, Sonic's speedy platforming, Knuckles' treasure hunting, and Gamma's shoot 'em up gameplay. (only now with Eggman in a mech)
Shadow, Tails, and Amy were all going to be non-playable characters. At e3 2000, the game was announced, with the trailer I posted above. The game showed off Sonic, Knuckles, and Eggman, but Tails was nowhere to be found. Fans complained about the lack of their two-tailed fox friend, so SEGA decided to add him with a copy of Eggman's gameplay. With Tails in, it messed up the idea they were originally going for. Having Sonic, Knuckles, and Tails in the story didn't make sense. So they had to add two more characters to Eggman's story. Shadow was an obvious choice to take Sonic's gameplay, but this forced them to find a new character to copy Knuckle's gameplay. Thus, Rouge was born.
Now, there were a few problems that arrised with this. First of all, the story was originall going to run on morals, similar to Shadow the Hedgehog, this had to be cut. I'm thinking that SEGA originally was going to use Shadow as Sonic's dark side, rather than a whole different character. When Sonic did something bad, he would turn into Shadow. Again, this is completely speculation, and I may be wrong.
The second problem with this is levels; SEGA was probably done creating the levels for the characters. When I played this game when I was little, I always wondered why there was a lack of Shadow levels, only three! This is probably because SEGA had wanted Sky Rail and Radical Highway to be levels for Sonic. (as shown in the e3 trailer, Sonic's on Sky Rail) They added White Forrest because they needed a new level, and it worked with the story.
Eggman had to give up some levels to Tails, and Knuckles gave up some to Rouge.
So, what do you think?