If you want to shell out for an additional kit, you can make a REALLY long Green Hill.
Earlier this month, Toastergrl, the original pitch designer for the LEGO Ideas Sonic the Hedgehog set, offered optional instructions to add some extra articulation and detail to the Dr. Eggman figurine. Now, she has returned with optional instructions for four brand new modules, an Eggbot, a Chopper, and a Buzzbomber.
Be aware, building this will require you to either take apart most of the existing LEGO set in order to make, OR buy a second set if you want to combine these modules with the original. It also gives the model some dimension, adding a background platform, and an angled bend.
Source: The Brick Fan