The different themes are marked here. Read below for more ideas.
A1: Forest & temple gate
Enemies: Dragonfly, temple gargoyle.
Scenery: Temple gate left and forest right. Exit in temple. gate.
Enemies: Bird, Armadillo.
Scenery: Branches, vines, grass.
A3: Forest and house lawn
Enemies: Ant, mole.
Scenery: Picket fence, mailbox, garden tree, house entry.
A4: House kitchen
Enemies: Toaster, knife, mixer.
Scenery: Owen, sink, microwave, fridge, drawers, table, pet food.
B4: House livingroom
Scenery: TV, NES, sofa, paintings, lamp, bookshelfs.
B5: House office
Scenery: Computer, drawer, desk.
Enemies: Chicken/Cock, buzz saw.
Scenery: Hayloft, 'hay cubes', logs.
Scenery, Cellar junk.
Enemies: Slimes, steam leaks, wrench.
Enemies: Sparks, Acid, mutant.
Scenery: Tubes, potions, wierd lab machinery.
Enemies: Manufacture robot, cleaning robot, guardian robot, stomp/print pistons, grab arms.
Scenery: Tracks, elevators, parts, assembled parts, finished goods.
A7: Power generator
Enemies: Spark wisps, elec beams.
Enemies: Loader robots.
Scenery: Elevators, crates.
Enemies: Spike traps, Crush traps, ancient robot.
Scenery: Pillars, buttons, roots.
C1-D2: Ice caves
Enemies: Falling ice, yeti, penguin.
Enemies: Bats, beetles.
Scenery: Brown rock.
E4-E3-F3: Lava caves
Enemies: Lava balls, fire bursts.
Scenery: Brown rock, lava.
F4-F5-E5: Alien lair
Enemies: Alien, floaty alien, tentacles.
E1-F2: Boss area
Enemies: Skull, skeleton, ghost, zombie, reaper, spikes.
Scenery: Chains, gothic, heavy metal stuff.
E6-F7: Bonus screens
Enemies: various stuff that got left over.
Scenery: ditto, like... desart, junkyard...
There are a bunch of possible enemies we can create, the above is provided as inspiration.
Please feel free to create animations for any of the above enemies (or any enemies you might imagine). I'm sure that if you do so somebody (maybe me) will create a script for them so they can be in game.
Some things to keep in mind are palette consistency, and art direction/style.
I believe Arne may still be working on the palette, but if you keep to the same colors as in the original, you should be fine (color conversion can happen later with ease as long as the original colors are adhered to).
I'm not a great pixel artist, nor am I a good animator - I'm sure there are others like me reading this. So for you guys, I suggest we take dibs on scripting enemies as they are posted.
If it seems to be taking too long, I guess someone will post here and ask you to respond. If you don't respond, that person can take dibs... I guess?