I just played through the demo and very nice work very fun and entertaining.
I did find a few issues one the flurries are kinda buggy if you have a fire flower you can't pick them up as you shoot a fireball at the same time and fry their poor little fluffy selves and second they don't have great physics with heavy gravity like the fall super fast but this also allows them to clip through slopes and just fall out of the world.
The second issue is in the castle during the water section when it's all the way up what looks like safe ground actually kills you.
In the screenshot the large ice block looks like it could be stood on but if you stand on it, the game counts Mario to be in the water and you just die.
Lastly the boss I guess it's not really a problem but could you edit how fast he shoots the magic/fireballs? like not the frequency but how fast they move cause they're a little too fast.