I'm probably going to regret this. I'm no game developer and I don't know shit. I usually talk out the ass and end up sounding stupid, but here goes.
I liked them as a kid. Nowadays I'd say that it was a waste of development time. But I also think that variety is something that Sega games have always had, and it did extend the longevity of what would be a platformer that you would likely only play again to learn the levels better. I'd be fine if they showed up. If another Adventure style game happened, and they felt they could do it without sacrificing the quality of the main game, then sure.
It depends, I guess. I would really like to see them try to build off of the first Adventure. Perfect it. I'm not sure how this relates to "pace and speed" exactly. The classic Sonic games and even the Adventure games did have a lot of subtlety, contrary to popular belief. There just needs to be a balance.
Random question, but I don't really think a 'Sonic Maker' is feasible, if we're speaking of classic Sonic. This will make me sound biased, but Sonic is more... I dunno, complicated than Mario. Mario is the perfected platformer. You can't really go wrong. All the mechanics are easily understood, and do the exact same thing every time. Sonic is different. There are more variables, and a lot of times, breaking the game is sort of the point. Learning how to use the hills and such to jump higher and go faster, for example. Sega/Sonic Team hasn't really proven they know how to make a classic Sonic game anymore in the first place, let alone have the tools to make an editor. Mario Maker was built on existing tools and programs that the actual developers used before. I doubt Sega has this. If you want fan created content, there's plenty of really great fan games and ROM hacks out there.
Of course I want a story. Sonic games used to pride themselves on having more context and world building. Sure, it wasn't great all the time, but it was a ride. Like a cheesy movie. Eggman as a bad guy is fine, but there needs to be a good reason for it. At the risk of parroting Roger van der Weide and others, I want to play as a character in an adventure, not a vessel for me to jump around with. That's kind of what I look for in video games in general. It's incredible that lately Mario characters, the most video game-y caricatures ever, have been more animated and full of personality than Sonic characters have been for a long time.
It's not really a simple question. Like I said, if the developer thinks they can handle it, then yes, definitely. More variety means a better game. But they always seem to spread themselves too thin. I'd kinda like to see more experimental spin-offs starring Sonic's extended cast. There's a lot you could do there, I think.
Context is an important factor, here. In Sonic 3, the short silent cutscenes served to show where your character was going in the actual world. It felt like you were traveling across the island. Adventure's hubworld served the same purpose. It was a little clunkier and slower, but it was fun. A bunch of stupid stuff to do. Again, the developers need to actually think about this. Connecting the levels through cutscenes would streamline things, but a hubworld would extend playtime and add more, y'know, variety.
Again, the developers shouldn't bite off more then they can chew. This is the more ambitious Sonic games' problems.
I want a big grand adventure, but a story can make it feel 'big' too. Gameplay is very important, and that should be the focus. But a competent, well-written story, made in tandem with the gameplay, can make the game feel grand and epic. I just am not sure Sonic Team can do either of those things. The last game Sonic Team wrote was Runners, and it was so bare bones and full of those stupid translation things that kept showing up in the Adventure games. It reeks of indifference. It's a stupid low budget mobile game, so maybe it's not indicative of anything, but still. Even that, though, would be better than the writers of Colors, Generations, and Lost World. They suck the life out of everything. They need to go back to doing contract work for random cartoons. I doubt they have great working conditions, Sega probably just hands them a piece of paper with basic guidelines, but they need to bring in somebody who cares if they're not going to do it themselves. The current writers barely have basic knowledge of the series. (Apparently, their 'research' was watching cutscenes of the games on the internet.)
Woah, that was long. That last bit kind of went on a tangent but eh. Tried to speak rationally. Hopefully someone can glean information from it. Also, wrote this before you added the extra stuff.