I love this game. I played the Dreamcast version as a kid so many times and recently replaced the PC version.
I learned that you can unlock metal sonic after 100% completing the game, but he is just a cosmetic for sonic and thats it.
My problem with this is that once the game is 100% complete, do I want to still play just to see a different cosmetic?
Maybe if there was some variety to make the levels a bit different or if he was able to be used after getting all the B rank emblems so there is still some challenge to complete.
You should look into the Dreamcast conversion mod and the Lantern Engine (lighting engine tweaks) at the very least. Pretty much everything that comes with the “Better SADX” modloader is great.
In short, when they updated the Dreamcaast original to DX, they changed the textures entirely along with most of the player models and broke the lighting engine. Every time the game was ported after that, the textures have gotten more compressed and lower quality, and more bugs have been introduced.
For example, you know the spot in Emerald Coast where Sonic can clip through the wall really easily and drown? Never happens in the Dreamcast original, fixed by the conversion.
So mods that restore stuff tend to bring the quality up considerably and fix bugs. Personally I prefer the DX player models (the Dreamcast ones look barely better than PS1), but you can adjust the DC conversion mod to switch off whatever changes you don’t like.
Oh, and for replayability, the time trial ghosts mod is great. It displays a see through playback of all your movements from your fastest playthrough of the level, like racing games do. So you can race yourself.
Also, with the modloader you get real widescreen. No more inane borders on the side squishing things to 4:3.