What kind of features would this offer over just using something like Sonic Worlds in MMF2? Aside from being free I suppose, here's the main things I'm looking for:
The sprite editor being able to work very fast, importing sheets or frames easily and handling placements of pivot points with common sense (MMF2 has issues with this).
A quality level editor that doesn't have the petty limitations other game dev tools tend to have.
Quick placement of common objects like layer swaps and spin triggers.
The ability to create prefab objects (connected groups of objects that can be instantiated at once), such as a badnik with multiple collision sensors, bosses with multiple parts, or even copies of the entire player with all of it's needed objects. This would be a vast improvement over how many other game dev tools work, especially since any such softwares with Sonic engines do not have this.
Using DirectX? Have some 3d rendering power in there. That would be a huge advantage.
Level editing oriented for Sonic games for fast workflow:
Tools and widgets for unique properties of common objects, like the monitor type, spring power, size of triggers, drawing lines of rings, etc.
Visual guides that show various relevant details, like the launch path of a spring (the path would assume lack of input), or a set of guides that follow the mouse that shows Sonic's jump height and approx distances based on running speeds. Things like that would be tremendously helpful.
If you're planning to use a tile and chunk based level system, for the love of god don't make it a cumbersome workflow like rom hacking tools and such. Make sure the system also supports freely importing images of arbitrary sizes and placing them right into the level. Also, supporting direct use of the alpha map as collision, that is important for both level elements and interactive objects. This program would also have a huge advantage by supporting drag and drop of images from explorer right into the program for instant level graphics.