Actually there is one in any version of Sonic Worlds... Anyway... Lake made some guides for Not-so-simple Sonic Worlds here: https://www.youtube.com/playlist?list=PLDUvAL48k3SDoXeGJMEDErAnBdbGvjoLF. Don't know if it includes a boss toutorial...
I think you might have a cool concept in your hands! A small thing I would do is recolor the Special Stage Sonic sprites using the Mod.Gen palette from the regular Sonic sprites you use in regular stages.
Uhm Pootis, lake used these spark tiles in his own version of sonic worlds so DG or whatever his name is got them from there likelyThis is going to be hard to describe this in words so let me first show these images:
What I'm trying to say is that there is some really bad clashing in these screenshots. I can't even make out what you're trying to do with the theme of these levels (apart from the 3rd screenshot you've showed which I was able to identify as some sort of Metallic Madness kind of level) and leaves me with a bunch of questions.
Is the 4th screenshot trying to show a Casino level? If that's the case then I don't think the background for Marble Garden Zone would work here since Casino levels usually take place in a city of some sort at night.
Is the 2nd screenshot meant to be a grassy hill sort of level? Because I don't think the tiles from Spark's Network Harbor work for a grassy era (not even colouring them green would help here)
I could go on with these questions but I'll stop it here for now before I get too nit picky. I'm not trying to stop you from using existing tiles for your game but if you're going to do that then you should think about picking more appropriate sprites for the theme you're trying to do for these levels.
Also for the title you're doing, how can an "old adventure" also be "new" at the same time? Is this another New Super Mario Bros scenario of trying to make a game that will no longer be "new" in a few years look like it's brand new? The title doesn't make much sense when I look at it.
That's all I can say for now until you post the demo up at some point but right now I'm not really impressed with what I'm seeing here.