I did play through the 4 levels. I didn't find any secret exits and I'm certain I missed a lot
of stuff, but here's my input anyways!
I like that there seems to be a balance between linear and more exploration-like levels/segments so far. I think that formula can work well in your hack, with more linear levels working as "breathers" between the more extended exploration adventures. I really enjoyed level 1, it's a quite fun, simple platformer.
Levels 2 and 4, which are probably the highlights of your demo as they show off the design you're going for, were mostly cool too. However, I'm afraid things have gotten a little too hectic or cramped at times. I think it's important to keep the platforming not so precise or tight in exploration levels, as looking around, memorizing paths, etc. is already a great challenge on its own. In particular I'm not a fan of how slow the venus flames are - I think that's the default speed of the current sprite we have, but yeah, I really don't like it, I always tweak that one - and the roto disc crazy patterns are a little too much sometimes - I got sniped by one of them before I had any time to notice its presence (a very fast one in a segment with a bully).
Also: try to avoid forcing backtracking. A glaring example of that is the dragon coin after the midpoint, in the castle - which forces you to take a gamble with the switch, and if you want the dragon coin, you'll need to return again. The difficulty of the jumps to get to the switch only makes it worse and more stressful! There are ways to make exploration flow nicely, not punishing the player for making wrong guesses or forcing them to replay segments over and over.
e: something else I remembered: work a bit on your powerup distribution. Make sure to give the player powerups in a regular basis, depending on the challenge, and appropriately give powerups after a checkpoint.
I'm sure difficulty and the like will be much easier to evaluate once the multi-midpoints are working, though. Once that's settled, you'll be able to get away with making challenging segments without making it too overwhelming for the player.
I don't know if you already have plans, but: how about one midpoint per collectible? They're sort of an ultimate goal after all, and it would make sense to save progress once you get each of them.
I'm not sure how much what I wrote helped, but I hope it did help a bit. It felt really good to play this overall, and as I imagined, this feels like it has a huge chunk of potential. Best of luck with this!! I look forward to seeing more from you in the future.
Legião Urbana - A Tempestade (1996)