Cel shaded goodness for SA2, changes lighting of player characters, enemies, non-stage-specific objects/items in all action stages and both Biolizard Fights. Planned to be tweaked/improved and extended (especially to other boss battles) down the line. Works with MOST if not all other character model and texture mods as changes different files. Okay technically it's closer to 'flat' shading than Cel right now, also dark areas in many stages give player characters a green-ish glow, this is W.i.P incarnate.
If you have Iron Gate Lighting Fix installed ( https://gamebanana.com/gamefiles/8867 ) make sure to deactivate it to use this mod in Iron Gate (it's perhaps the only mod I know of that edits the same file types).
As you can see big gains can be had with other mods, this is the HD Modern Models Pack by which can be found here ( https://gamebanana.com/skins/172923 ).
There will be errors especially some stages in shade, I had to redo files with artificial chaos enemies in and might have missed the odd similar bug. While this project is in alpha state I do intend to keep updating and any major alterations (for example texture changes which would impact other mods) will be keep as optional additional downloads.
Finally obvious best-fit for this mod would be Jet Set Sonic ( https://gamebanana.com/skins/173798 ) by Lady Luna - download it in addition to this mod for no regrets.