Dang this is becoming a really good mod!
Unfortunately, I have to leave in a few minutes so I don't have time to post comments but I will as soon as I can!
EDIT: alright comment time!
-Some custom music would be pretty sweet.
-I love how you put secrets even at the very beginning like the robot who gives you the expansion pack.
-I'm very glad you toned down the damage on the Curly/defense grid bullets. It's much more reasonable now, especially since it's the first boss in the mod.
-The moving block thing at the bottom of the orbit place was awesome.
-Grass growing on metal blocks looks quite odd and out of place. I think you should use a different tileset for that map in which the metal blocks are replaced with something a bit more organic looking.
-I love the rotten roast's sprite, although the fight itself is just m'eh.
-Autothresher is really fun.
-Is it possible to get to the computer that unlocks the door without bat/bug hopping? If it is, ignore this comment. If it's not, make it so. Requiring the player to "cheat" like that is really unprofessional.
I haven't gotten past the action level on the other side of the door yet, so that's all I can say for now. Keep up the good work!