Luigi's Super Adventure - World 6 Done and back again!!!

I'm gonna have to agree with Katerpie in that I feel the levels are very similar in design with similar obstacles appearing around the same time. The only progression evident from one level to the next is a new threat identified with the fire-spitting Piranha Plant. You do have a solid understanding on how to make a level look nice: it's very clean presentation. However, using the same BG and palette as the prior level only adds to the feeling that it's 1-1: Part Two.

If you modified Luigi's physics to make him more lofty and jump higher like in the later Mario games, then I'd recommend setting up a test level so you can get a feel for how to space your resources. You always want to build the levels around your character's physics. (What you have now feels almost like it could be played in SMB1 if not for slopes, and that does not pair entertainingly with SMW physics, and it would certainly be much too easy with updated Luigi physics.)

Whereas you like to use the red Parakoopa, I think you only using him over pits where he can easily be bypassed as a non-threat and non-asset is wasting Luigi's air time. Make it occasionally mandatory to have to hit the Parakoopa to cross a pit or reach a secret. Parakoopas are nearly limitlessly exploitable in everything but water. The shells of Koopas themselves can be used to do so many things you can't conventionally do otherwise (e.g. see 1-3 in SMB3 for a lucrative example, also clearing enemies with ease, taking Lakitu's cloud, or shell-jumping.) Other jumping enemies like the Piranha Plant offer more dangerous ways to elevate Luigi: for example, think about turn blocks blocking off some reward you have to be big to get, but you can only reach it in the first place by spin-jumping off of a jumping Piranha Plant. These are just a few ideas to spice up your levels. The more you experiment with resources, the more you can think of, and you will find yourself revising levels again and again until it feels just right.