*note* it's more like a tip not an Easter Egg and I didn't play this game for a long time so I forgot some of the terms.
Also, trust me on this, use Team Rose, don't be brave and use another team, unless you are a complete Master.

All 7 Chaos Emeralds:
(1). Beat Team Rose (Amy, Cream, and Big) Story first.
(2). Go to the menu where you redo missions (not story mode).
(3a). Go to Team Rose and do the collect 200 rings missions on the stage before the boss.
(3b). Make sure you get the Key and don't get hit with while holding it.
(3c). Complete the mission and go to the speical (wrong spelling?) stage.
(4). Stay on speed (Amy) at ALL times and start pressing square when your gauge 1/3 full.
(5). To the chaos emerald, you need to kepp up with it and get past it.

The Final Story:
Once you get all 7 emeralds, complete the other stories if you didn't. If you did you be able play it. You don't have get them all at one time if you don't want to.

Hard Mode:
(1). Beat every Story including the final one.
(2). Get an "A" rank in all 120 missions (don't worry if you don't get an "A", you can always redo it).
(3) After that I'm not sure (I only got to 115 "A's"). If you get to 120 wins please message me or comment me.