It's the sprites themselves that are troublesome, for three disparate reasons. One, there's probably a bunch of poses that S3 needs and Mania doesn't, so someone would have to draw some more colorful mice (no thanks). Two, and this is related to three as well, Sonic Mania has a lot of extra sets of animation, like the walk/run poses are now at least 3 or 4 different cycles, there's a version of the walk cycle used exclusively while falling and probably more stuff I haven't noticed. Worst case, these would just be simplified and omitted where not really necessary.
Reason three however is the devastating blow. Not only does Mania add a lot more cycles, but it adds a lot of distinct frames to each cycle, and we're already up to the 250-ish frame limit of the S3 animation system. We would have to do a signficant overhaul to the code and data structures to add all the extra smoothness, and even then you're still running up against missing S3 poses, a poor color palette, and hey now you just filled half the ROM with uncompressed character sprites. Might as well stick to the mantra of "if it didn't happen before 1994, it didn't happen". (except for mobile remakes ok)
That said, if you can expect anything, then definitely expect some nods to Mania, in perhaps more meaningful ways; I at least will certainly get more inspiration once I've played it.
Thanks for showing interest in the sprite options though, it's an occasionally reviled feature that we (I) have to maintain, so it's good to know some people care about them. I've been toying with some old sprite ideas which have now become tangentially related to Sonic Mania, so feel free to be curious and excited about whatever that means.
Hey, yeah, I too really appreciate all the sprite options! My favorite of all are still the new Super/Hyper sprites, and I hope we see a Sonic 2-style variant eventually!
If you're implying what I _think_ you might be implying, then yes, getting the Drop Dash as an option would be really fun! I couldn't really care less about Mania-style sprites, though. I'm not really a fan of how aqua blue Sonic is there anyway, and there's no real reason to recode S3's structure just for smoother frames.