If Bowser stole the one and only Golden Mushroom (TM) why are there a bunch of golden mushrooms just lying around on top of those metal totem poles?
Anyways world 1-5 needs a serious BG change. The background blends wayyyy too much with the foreground. Without actually playing the demo, I like how some levels look, while others could use palette changes and more varied use of vanilla sprites.
Still, props for having interesting world themes. If you are already making a christmas and halloween one, it could be interesting to include more worlds based on holidays/seasons.
e: main issue with palettes is heavy saturation in combination with gradients and having no contrast. Don't use the Lunar Magic gradient feature, just try messing with the colors without picking such heavily saturated versions of every color (for example the green you use is like intense green). Use the original smw palettes as a base and change them slightly until you are satisfied and they vary enoigh from the original.
For the BG issue, I think it would just be better to remove it altogether and think of something new unless you can find a way to darken it even more without it looking dumb. The rest looks brilliant by the way, there's only a small lack of enemies.
You should probably explain within the plot what is so important about this golden mushroom. As it stands, all I've gathered is: It's a mushroom. It's golden. Bowser stole it. So then, why does Mario need to retrieve it?
RussianMan already addressed the other nitpick I had, but I'll chip in and say that it really is something that needs fixed.
It does. Details which would normally use their vanilla graphics in the FG look confusing on the BG, but only when they use the similar/same palette as FG. It's not a castle level so it's not a problem if the castle tiles are not used.
These screenshots you've posted are pretty good. My question about screenshots 3 and 6 is, are they in one same level? I understand the cave tiling looks identical, but I don't know if the background changed. I can tell the difference that the pipe colors on the two screenshots are different.
Screenshots 4 and 5 need to be fixed for the following reasons:
1 - Both screenshots have the pink treetops covering the life counter, star points, reserve power-up box, timer, coin counter, and the scoreboard.
2 - Screenshot 4 has some sort of cut-off with the pink treetop on the top right corner of the snapshot.
I highly suggest fixing what I pointed out in your screenshots.