Sonic Colours was the second Sonic game in a row to feature only Sonic. But it also introduced a whole host of other changes:
- Different voice actors. (Mike Pollock had to re-audition)
- Different writers.
- Lowered difficulty.
- No hub worlds.
- Red rings.
- Mario inspiration (in Sonic Colours and Sonic Lost World).
Wisps were to compensate for other gameplay styles. That’s why they were introduced to the gameplay in Sonic Colours, and remained also in Sonic Lost World despite holding zero plot relevance there. (Sonic Unleashed and Sonic Generations had alternate Sonics instead) Iizuka stated that they were to be a standard for the future games.
There was however one instance in which Iizuka said that he had thought of making a Tails game and a Knuckles game in that past, and that he actually considered making a game like Knuckles Chaotix at one point, but SEGA didn’t give him the green light to do so.
Perhaps this standardisation was why they used Sonic Boom as a sort of sandbox for testing other characters, but the problem was that Sonic Boom wasn’t even close to any of the Sonic games even when they did include other characters.
Iizuka wanted such stories: Sonic Stadium
SEGA wanted Sonic to sound older: Sonic Stadium
Yuji Naka finds Sonic games nowadays too easy: gamesindustry.biz
Wisps were to compensate for other gameplay styles: Sonic Stadium
Wisps to be a standard: SEGA Nerds
On page 101 of gamestm #100 issue from 2010, Iizuka says this: “Sonic has always been a character focused on speed and speed alone. If we wanted to incorporate some other type of gameplay into our previous games then we had to introduce a new character who would give the game a different layer of action. With the newest Sonic games, we’re going back to the fundamentals of having Sonic the only playable character, so the games should concentrate on the high-speed element. At the same time, however, we want to feature those different layers of action, so that’s why we came up with the Colour powers.” “The Colour concept is one of my favourite features in the Sonic franchise, and I think the team should be really proud of it.”
Source: TSSZ News (also in the article itself, you can see that Iizuka started standardising things when Naka left)
However, in an interview in August 2010: “NiGHTS isn’t the only Sega franchise that Iizuka is eager to revive. The veteran Sonic Team director was also behind the generally disliked Shadow The Hedgehog and has a surprising answer when we ask him why he’s never made a dedicated Tails or Knuckles game. ‘I’ve thought of both in the past’, he reveals, ‘and I actually considered making a game like Knuckles Chaotix at one point, but Sega has unfortunately not given me the green light to do so.'”
Source: Sonic Stadium