I would actually disagree, I found the level to be pretty boring, to be completely honest. Yet, the aesthetics are
amazing! I wouldn't worry about the background because it's still really awesome what you've managed to make out of vanilla gfx. Have you tried using an invisible sprite C1? Even if the background isn't moving fast, the gfx in this level is really interesting. It's also cool how you used coal as a sort of gameplay gimmick as well!
Here's the problems with the level.. it's an autoscroller, which means you have to design your level accordingly, and so far it's just flat and boring. So I did a bit of testing, and switched the "slow" auto scroll to a medium auto scroll, and let me just say it's already way better. It helps the slow sections go by a lot faster, and just keeps the level interesting. I would
really recommend switching it to medium auto scroll. Also, I would steer away from hiding blue kicking koopas in the coal, as it is already a really annoying enemy, and hiding it is even worse.
As for the enemies, I would say keep the diggin chucks, but if you're using the Mushroom sprite tileset, there's tons of unique setups that you can take advantage of. I would use the ground guided fuzzballs, the moving checkerboard platforms, and maybe some other sprite with tileset mixing. Use the rocks without the chucks, so you have to spinjump on them. Use tile 1f0 (even though everyone does it) to put floating rocks in the level, which Mario has to spinjump on. Since the level is autoscrolling, it has to be highly linear, which in this case I think the best option would be to concentrate on platforming. This means less safe places to stand, and using the floating checkerboard platforms, moving rock platforms, etc. to create a less static level and a constantly moving one, which keeps thinks interesting. For a level name "the train is late", I felt that the player should be more frantic and rushed. Not
too rushed, but just enough that you're constantly interested and "on your toes".
-Have the message box tell players that the railings kill you. Also, this room is empty, which is fine, but the room prior to it was empty as well. It's a lot of emptiness in the beginning. I would maybe introduce a few gameplay elements? Perhaps the coal can be introduced here, as something that obstructs visuals? Well, on that topic...
The coal gimmick itself is iffy, and I think something more interesting can be done with it. That's really up for you to decide, but maybe you could concentrate on moles only popping out of coal? Do something more interesting with the coal at least. I would also find it interesting if the train was split into sections? Like, a boiler room with lava, where you have to hop from coal piece to coal piece without getting roasted by hopping flames or podoboos (in this case the "coal" would be the brown platform that floats in water). Also, maybe a dining section with koopas and chucks that are trying to enjoy a meal, but they get disrupted and try to kill you. It sounds tricky to make, but I feel like it's possible.
-This section is awful. It's not fun and it makes no sense... if you want to introduce the gimmick of Chuck throwing coal, I would do it in a safe environment. You can't simply throw the player into a chaotic situation and introduce a new gameplay element. I would introduce the coal-lobbing Chuck dude in a totally safe place, where you don't have a chance of getting hurt. I would greatly expand on the interior section as well, there needs to be a lot more stuff and, since it isn't autoscrolling, gives you the ability to switch from platforming to exploration-based design. This is important.. you don't have to design the entire level the same way, since you can switch from inside the train and outside.
-This dragon coin is way too easy to get, I would recommend making the dragon coins require at least some effort. Not too much, of course.
Anyways, I hope this helps. The atmosphere and aesthetics of this level are really phenomenal, and it needs equally level design to match, in order to be a top scorer and contender for the best world.
I just don't know how to make a gif of that from game footage, so it's not in any pics!
