That submap is FAR from finished, it barely has any decorations and there's no land structure whatsoever. Raise and lower some land, add water and don't use trees in a cave... If it is a cave atleast. I don't know what kind of area this is, but I guess it's a cave or ice area.
- Use event for your paths, which will be revealed after finishing a level.
- Don't leave it flat like this. Build some rivers, lakes, mountains, etc.
- It's too bland so far. You have to put some more decoration. Mountains and trees is a good idea though.
- Maybe try some perspective? Some mountains, waterfalls, etc. Just take specific care about the perspective, to avoid making perspective errors.
- I think those sparkles don't fit on the top of these mountains, but putting them on the cliff edges may look more fancy, like SMW used them in Vanilla Dome.
Think a little creative. Imagine yourself travelling through the world and passing through various obstacles. It may look nice on the OW. Also you can draw the OW on paper, it also sometimes gives people ideas.
I'm going to separate this into The Good, The Bad, and The Ugly. That should make it a bit more clear what exactly needs fixing.
+The palette which you used is very clean. I like that bluish hue, and overall, I think it works rather well.
+Assuming it's an icy cavern, I think the trees are a lovely, creative touch, which can be used for some excellent level design opportunities. I strongly recommend you keep those trees, or at least the idea thereof around.
+Those sparkles are basically pointless. They are very distracting when you look at the design of the area.
+The palette of unlocked paths and the pipe. This clashes against the subdued blues that you used everywhere else, and it just feels messy to me.
+Nothing is happening. Look at Vanilla Dome from the original SMW. What you'll find is that there's a lot going on. Mario walks on multiple levels of elevation, there is a small lake, the sparkles feel well-placed rather than just sprinkled all over the map and Mario's path has obstacles which actually make sense. Whereas your submap looks... bland. It's all flat, Mario's path feels like it could go anywhere, and really, there's nothing interesting.
+It looks like you hardly put any effort into this at all, being honest, save for good palette use.