Front-loading this with some of my unwritten game design philosophy™: I believe the fundamental basis of fun is tension, which is essentially the threat of failure. The key to understanding any genre is to identify its primary action, the means by which it generates tension. The primary action of the platformer is jumping; the conflicting force is gravity. The more jumping the player has to do, and the more pervasive the threat of gravity, the more fun that game is going to be. I think the reason we generally still hold Super Mario 64 to be the best of the 3D platformers is due to how inherently vertical the level designs are. Verticality is exhilarating because the risk of falling becomes increasingly more costly- more tense- the higher you go. Most of SM64's maps are essentially a 3D bell curve- fairly open around the perimeter with a large elevation at the center. If you're shooting for open-concept level layouts, this structure is really the optimal way of going about it, since it leverages freedom and exploration with a sense of direction. The goal is compelling to the player and visible at all times.
The problem to be solved here is that Sonic's speed clearly prioritizes horizontal movement over vertical traversal. Horizontal tension is racing genre territory, and that works just fine for the Unleashed formula, but the point of bringing classic Sonic into 3D is to emphasize the platforming. As a baseline, when you're designing these maps, time out the spacing of your geometry so that the player never has to go more than 2-3 seconds without jumping- unless you're explicitly doing a run-for-fun segment (because pacing matters too). This will help you work out the density of design you should be shooting for, which is definitely something to focus on, because a lot of this map is just outrageously flat and boring. Bottom line: Jumping Is Fun. Speed only matters insofar as it enables bigger and better jumps.
Of the two games, I think Green Hill Paradise has the stronger level design- far from perfect, but it's doing a much better job of marrying Sonic's physics to the platforming. It's extremely vertical, there's a good density of iconic landmarks that each represent a puzzle to be solved my means of momentum-based platforming with a tangible reward at the top (Chaos Emeralds). Oldschool Cliff Bleszinski wisdom, one of the cornerstones of good level design is showing the player a visually distinctive landmark and making them think, "Awesome, now how the fuck do I get up there?"- something teased off in the distance, just out of reach. They may not even realize they were working their way up that structure until they get sidetracked and just happen to find themselves at the top, where they're rewarded with a moment of epiphany. GHZP does this wonderfully. The problem is that it controls like shit, and you become so singularly obsessed with scaling these landmarks that you wind up trying, failing and backtracking over and over, which is profoundly un-Sonic. Something we need to understand about the classics is that though they seldom explicitly prevent backtracking, they do discourage it- it's easier to go down a slope than up. Higher paths are more rewarding and require more intensive platforming, but when you fall, it's always more interesting to press forward. Save what you missed for the next playthrough.
To this end, I propose taking SM64's bell curve and inverting it. The Zone 1-1 of 3D Sonic should probably resemble a bowl in profile. You want to talk about easing the player into the classic Sonic state of mind? Make it a Downhill Jam. Punctuate with plenty (plenty) of flat ground and peaks to climb, but if they fail that climb, don't allow them a second chance. This ties into a critical element that most every 3D Sonic to date has largely been missing (even SRB2, while still having the best level design to date, doesn't push this far enough due to technical limitations)- floors over floors. At peak density, I demand your level have at least three layers of ground overlapping one another. Float your geometry. Player needs to play smart to leverage enough momentum to reach the highest levels, and the layout should feature multiple opportunities for them to filter up and down through the stack. Merge back down to the ground plane at the end of the map.
Going back to verticality, I think the route to making verticality work for Sonic, and to really set the game apart from every other 3D platformer out there, is to really make his surface clinging a primary focus of the game. You have the helix and at least two cosmetic wall runs going- I want to see that become a staple, and I want it to be a fixture of the most challenging routes through the map. Do this, all the time. Allow the player to run upside-down. That's a feature we seldom recognize about the classics, you don't lose momentum when running on the ceiling. Sonic GDK doesn't account for this, but I think you should. Bottom line, it's okay to be this, but if we're faithfully extrapolating classic Sonic into 3D space, it's going to look more like this.
Okay I'm out of juice. This is the game to solve 3D Sonic and your reason for existing right now is to get this right. The importance of this cannot be understated and your soul should be reeling under the sheer gravity of this responsibility, but I think you guys can do it (and maybe I want to help someday because I really like level design and painting textures)