NIGHTTIME STROLL ZONE
MUSIC
http://www.youtube.com/watch?v=78mxBMKGIQo - But you know, with a soundfont to fit SRB2. Or something with a feel similar to this.
THEME
Act 1 starts out at dusk time. The sun is just setting, and a peaceful aura radiates throughout the zone. There are very few enemies about. For now, a basic crawla here and there will suffice, with one extra enemy I will detail later.
For texture choice, I should recommend the JNGGRSxx floors coupled with a diamond-ish version of the GFZ wall, but, you know, not so ugly. A dark-blue-ish colormap could be thrown over the entire level to heighten the effect of dusk time.
Act 2 is relatively the same, but the sun has completely set and the moon is shining full and bright. There are also noticably more enemies about. This is designed to be a beginning zone.
GIMMICKS
Trees: Yes, trees are a gimmick. Sometimes, to reach a high place, you would approach a sector-based tree with a zoom tube that would take you all the way to the top of the tree, ideally to emulate climbing a tree, so the zoom tube shouldn't move too fast. These could replace springs in a few places, just so the environment looks more ... natural.
Mud: Slows you down. Need I say more?
BADNIKS
Rhino Crawla: Remember that enemy in Emerald Coast from Sonic Adventure? He locks on to you and charges at you. He'll look like a combination of that and a crawla. Just ... not as big.
Tree Monkey Thing.: That robot monkey thing from Sonic 2/Adventure that hangs on a tree and tosses bombs at you. I guess it could throw, like, grenade rings or something. Obviously he would hang from the trees that you zoom tube up to the top of. Since it's a zoom tube, you'd kill him just by approaching the side of the tree he's on.
BOSS
Daddy Rhino: An upgraded version of the Rhino Tank, built to be able to actually combat Sonic. You can't harm him from the front, since he has that horn, after all.
Attack 1 - Dashes in a random direction and fires three missiles at you.
Attack 2 - Dashes three times, each in a random direction.
Panic - His dash and roaming speed increases noticably.
Panic Attack - A homing-dash. He locks on to your location and charges, for about 5 seconds. During this time, he shoots out 2 missile per second.
Arena: Kinda Bomberman-ish? A flat plain with an elevated ledge circling it, with boulders, trees, and the like scattered around. Good for taking shelter from his random charges.