I quite enjoyed your level. I found it to be quite unique in terms of its gimmick, what with the Chucks and Rocks coming out of the Coal, and the cave part you plan on doing. The level design was fun, and the aesthetics were done fantasticly. I'd say you got a good level on hands.
Now, you're looking for ideas on the second part, right? Well, you've got the Monty Moles, why not perhaps make them pop out of the coal, too, since you've got that gimmick already going on? Swoopers may also work, if you can use them, since it's a mineshaft/tunnel. And of course, use the usual Pitchin/Diggin Chucks and Bouncing Parakoopas. As for how to design the level, you're on your own.
As far as the problems with the level design so far, the others have covered most of the things. I do have a couple problems myself.
These coin chains were placed too low in my opinion. It's easy to miss the platform while trying to get all the Coins. I'd raise the coin chains up by 1 tile like the others.
And, to make it so that the cart looks like it's moving in the second half, I'd ExAnimate the wheels to turn around a bit. It couldn't hurt.
This is all I can help you out with, so I hope you can finish the last part of your level and submit it in time! Good luck!
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