Amongst that already mentioned....
Get rid of the Red Gem. (Why would you want to get a faster time by slowing your character down? Especially as that character is Sonic? You know... 'Mr Speed?')
'Adventure fields' fixes. (Everything should be available at the end of the game from there, all Action stages (All.), all town Missions, all field to field gates. (Why is one always closed?)
Silver Medals/Abilities fixes... So Silver Medals are supposed to encourage you to explore. In the game as is it is more of needle in haystack affair, at least to me, at the very least a chore rather than fun. Yet our abilities to explore are nerfed. Who here did not think... 'Neat...' when the got that Adventure field Emblem in Adventure1 by spin dashing on the roof of Tails Workshop to get to that cliff ledge? Yet the spin dash in this game..?
Unlockables for Emblems ala SA2:B. (Chemical Plant Act1 anyone?)(...Acually with this game mechanics... Never mind.)
Fix the timing on that Flame Geyser after the springs on flame core so you have the ability to dodge it through skill rather than luck.
Bounce attack and Light speed dash are different buttons.
Change the game so everything that was in the manual is in there, or change the manual so everything in there is in the game.
Some kind of explination as to what the hell the 'test of love' thing was all about. (Logic is telling me just get rid of it, but my morbid curiosity is demanding to know what the hell they were thinking.
Make Omega sound like Omega, rather than a talking toaster.
Change Shadows ending. (The whole taking off the arm bands deux ex machina is a SonicX thing right?)
Change Silvers ending... kill him off. Painfully. With no chance of him pulling a Shadow and coming back.
If your going to shoehorn an *intresting backstory development !!* to a character, at least make it make sense, or clever.... or at least clear. (I am easily impressed and would have smiled if Blaze had fire powers in Rush due to Iblis now being in her... And her saying, 'blue hedgehog...' in a ever so mysterious way, just says despite the g=fact you insist on the series having a plot, you do not really know whats going on. (Shall we say this is after Rush... before it?) If you have to make a plot, make it like you know whats going on.)
Finish putting a fully playable Super Sonic into the game. (They clearly started at some point...)
Get some new ideas. Emerald Beach=Sand Ocean, Egg Carrier=Eggmans Battle Ship Ice Cap (Sonic Adventure1) act2= White Acroplolis Sonic Act1 Chaos=
Red Chaos Iblis, Sonic= Sonic Re colour Shadow Shadow re colour Silver Silver re colour Mephilis Literal Shadow re colour...
Or at least stop stealing others ideas...
Rescuing Princess Peach god knows how many times= Rescuing Princess Elise god knows how many times,
a furry called Fox chasing a train on MacBeth=a furry called Sonic chasing a train in Soleanna,
Rush Adventure=Zelda=Phantom HourGlass, Unleashed=Zelda Twilight Princess, Chip=Navi,
Clown Eggman Eggman Nega= Pirate Eggman Captain Whiskers= Elvis Eggman from 'TBA.'
...Sorry, we talking about a game or a series?
Edit; Egg Carrier playable as Sonic. (Even if it is just during the Egg Wyvern battle. I can not be the only person who was disappointed at the lack of ripping the ship up like the Sky Deck in SA1?
Some hint to newbies that light speed dash works in Mach speed areas. Most people do not have a clue.
Auto Save after Town Missions. Again, more for Newbies.
(Lives and 1ups, again for newbies? Relics of a bygon age?)
More Areas like Sonics Jungle act 2. There were multiple paths! Several of them! ...most were rails but.... Mutiple paths! More than 2!! in 3D Sonic!!!