Lets get right down to it though.
Realtime Global Illumination. The latest graphics cards can handle it.
Environment variables. Things like weather. Handling of a day and night system.
Any of the latest DX11 features that could possibly be implemented such as image based reflections, tesselation, etc.
An intuitive shader builder like UDK's material editor. I know that some people want to achieve certain effects but have no real shader programming experience. UDK's material editor is pretty nice and very robust.
Physics that stay true to a Sonic game. No floaty or drifty characters. Sonic is supposed to be fast and agile. I'm sure he's pretty dexterous with his speed so I doubt he'd be sliding everywhere.
I'm sure there are people who want certain things from Certain games. Some people are geared towards the classic while some are for the modern style of gameplay. Its frustrating when one chooses what he is going for but everything else is left over. I propose that movement handling and powerups be programmed modularly. What I mean is... If someone wants the classic styled movement, they'll use the classic movement module, script or whatever it may be. That way all the extra stuff isnt cluttering the game. I feel as though some people will either want everything done for them or just have a basic movement template they can build on. If you keep everything separate, it'll make it easy to choose what you want to work with rather than having to dig though code to remove things you don't want in your game.
I don't know much about the libraries out there. I do feel as though certain things should be implemented for immersion purposes. Things like low and hi pass filters for sound. The doppler effect is a must in a sonic game. Imagine zipping past things and hearing the sound react as it normally would.
Importing your assets is a big deal. Making your characters in whatever program you may use and exporting them to be read by an engine can be an excruciating task. Try to keep asset handling very open. Have a way to import all animations, materials, smoothing groups. FBX handles nicely but some people may not like Autodesk's FBX.
A simple "MAEK GAEM" button that compiles all your source into an executable or whatever. It should include your assets and whatnot.
This post has been edited by StreakThunderstorm: 17 February 2012 - 08:48 PM