Just started playing it last night since I've been waiting for the PC release.
Since the PC version was delayed, I took up playing Sonic 1 all the way to 3nK on a Genesis to prepare.
I gotta say, starting playing Mania, I'm slightly disappointed. Now, don't get me wrong, I'm loving the gameplay. I just feel that there were some decisions made that are preventing it from being my favourite Sonic experience.
I've only gotten to Studiopolis Zone and these are my gripes so far:
- Reused Sonic 1 and 2 level assets. The first two zones look jarringly bad after my Sonic 3 and S3nK play-throughs. I love the creative areas in those zones, but the fact that they weren't re-imagined visually makes the game look slightly primitive. Once it got to Studiopolis Zone, everything was amazing. Reusing zones the wrong way visually dented my experience a bit.
- Sonic's Sprite art looks worse. People will probably disagree with me here, but I loved the evolution of Sonic's sprites from S1 to S3. In Mania, it seems they made the decision to branch off from Sonic 2's Sonic sprite visually. The Animations are smooth as silk, but the look still looks like a step back from Sonic 3 (IMO)
- The Blue Spheres as a checkpoint bonus. Oh god, I don't even know what they give you, but after two of them I stopped caring. They take more time than the actual Acts and WTF, they're just the exact same ones from S3 and SnK. Terrible choice. That being said the Giant Ring Special Stages are awesome!
- General Visuals. When you look at each Sonic game individually, all the sprites look like they belong to the same design and each game improves on the last greatly. Mania doesn't do this. Seeing an Angel Island badnik in the same stage as a Chemical Plant zone badnik is jarring. It just doesn't match.
Now I love the new ideas in the zones and I love the surprises and callbacks to Sonic fandom in general, but the choices they've made that I've noticed so far have made it impossible for me to not see this as a Fan-game instead of valid next entry to the Sonic 1 - S3nK Legacy. My opinion might change as I play more of the original levels. The little I've played of Studiopolis is FANTASTIC in every way.
Now to answer the question in the thread title.
1. NO MORE REUSED ZONES. None of the original titles reused zones, but if you absolutely have to, then redo the visuals to a cohesive design.
2. Drop the Blue Spheres. They had their time. On that note, checkpoint bonuses shouldn't be long. Sonic 3's Bite-sized mini-games worked because they were focused on one or two areas of Sonic gameplay and didn't take too long.
It's a shame, everything I like about Sonic 4 is missing in Mania. Everything great about Mania is missing in Sonic 4.
I'm pretty sure Sonic 3 will still be my favourite Sonic game.