Howlitzer+
Not very creative, just a bigger, bulkier enemy. You even mentioned that there shouldn't be a new model.

Sandman
This doesn't make a very big difference. Also, again, not very creative. Just an, as you put it, "sleep colored" phantom with a different status effect.

Jelly Green Giant and Big Slime Cube
Not very creative, as usual. Also, really? 4x the health of the Royal Jelly? Also, the part you said they appeared in was where every healer would be in an arena. That's kind of crazy.

Giga
A good idea, though not very creative, it would be a very difficult enemy and would force people to learn how to counter them, as should any monster added to the game. however you said they appeared in place of every single 'mini' enemy in compounds. That means like the scarabs, dust bunnies, and slime drops, right? Yeah, no. That's terrible. You didn't mention them dealing any less damage than a normal devilite, so there will be tiny devilites (if you don't finish them off quickly while they are all in a huge group, you're screwed) throwing things at you which deal lots of damage in a compound. Not something I, and I'm sure a lot of others would like to find in a compound.

Haze Bomber
This seems like a really cool enemy, but why did you even mention a bomber if you're essentially turning it into a slightly modified knocker? Also, the chance of it randomly attacking is a bit random.

Sleepy Hollow
The sleep mist would be very annoying. Also, you said lumbers would appear everywhere a wolver would in a wolver den? Lumbers are supposed to be tough enemies, and fighting hundreds of them, as well as the Howliter+, all while constantly falling asleep, would be a huge pain in the butt.

Party!
What exactly is the point of the party theme? You even specified that there's only 'slightly' more prize boxes. Usually level 'themes' are supposed to represent the status effect being used by the enemies in the level, or at least the type of enemy being used. Since there doesn't appear to be a status effect, what exactly do Slimes and Devilites have to do with parties? Kleptolisks are in Treasure Vaults because the level is based on treasure, and Kleptolisks steal your items. Also, what does "Random family maintained" even mean? Not to mention, the confetti, with the way you described it, sounds really laggy.

Night Time Arena
Having 2 sleep phantoms would be pretty annoying, and what do you mean by "elite enemies"? I thought that was just the little thing you made up to describe the difficulty of your enemy ideas, but now you're saying they already spawn in arenas, and you said to add "elite" enemies, of which you only mentioned one, which is the big jelly cube. Also, replacing all moving enemies with turrets would get very annoying. You do know how many spawn at a time in arenas, right?

Also, about the optional part: The game shouldn't change this much based on your difficulty. The most that should change is that you get more items and crowns, not missing out on the ability to play certain levels. Also, what do you even mean by 'optional'? If you mean this is an optional part of your suggestion that the developers can choose to add, they can already do that with any part. And if you mean it's an option in-game where you can choose to play a different level, then why only on normal difficulty?