https://i.imgur.com/1Y0CVRV.png
Koopas could be designed closer to these variants from SML1
and for hammer bros
https://i.imgur.com/8xxYfxE.jpg
maybe something like this but with less spikes?
I like that idea for Hammer Bros.

Since they're supposed to be pirate Koopas, I was going to make their hammers into cutlasses, but they didn't quite look right when they were thrown in-game.
@pocket: OK, is it fully up to date now? I'll have another look.
Well that wasn't my intention, I just gave a general opinion based on the OP and you seem to have fans of the project so there's certainly an audience for it. I guess there is no "hd graphics" emulator option for SNES yet like there is for NES, well fair enough.
To be more constructive I can make a couple of suggestions:
-Try blowing up each sprite on top of the artwork but on a separate layer in photoshop or whatever. You'll see more easily where things are off proportion. One example is mario and luigi's noses which are too big in your version. They might be a tad small in the original but with such a low res one pixel makes a big difference.
-Look at the shading on the red mushroom, then the poison mushroom. See how the light source is different? I think everything would look better with a consistent light source, at least stuff in the same category of items or enemies like mushrooms/flowrs/stars, koopas, etc.
-For animations, try posting the gif as well as the frames so people can more easily see if there's a flaw.
-Looking at the bombs in SMB2 (this is the case for a couple of other sprites too), you've made them look flatter by removing the brighter shades, and non-shiny as well.
I could make some edits for these and various other things since that would be easiest to understand I think, but you'd have to post the sprite sheets.
I'll take these into consideration when I return to tweak some of these sprites but I don't think I can do all of it. I can't just trace the art in photoshop since I still have to work within the game's limitations. For example, I'd love to have Birdo hunch forward in a more dinosaur like pose like her artwork, but unfortunately, the game has her standing straight up, and since I don't know how to widen a sprite, I have no choice. There is simply no way to be 100% accurate to the art ( Which is a big part of why I think this project should have a different name ), but I would like to get a s close as I can.
What is it you'd want to edit? I was actually thinking of making .bin files of completed sheets publicly available in a MEGA link so people could help out and edit if they wanted to.
To be honest I'm kind of counting on getting a little extra help on this project, I'm fine editing enemies but the Mario sprites for TLL and SMB3 are starting to become a source of stress for me, and I haven't even started on Luigi, and as a result i've been having a lot less fun with this project.
EDIT: I've started a new poll to re-name this project. I've narrowed it down to 5 choices.
This project started with the goal to make everything look more accurate to it's artwork, and that's still the main goal, but I have a lot more I want to do now, like restoring elements that were removed or changed from the NES and famicom versions, making Lost Levels stand apart from SMB1, possibly adding some other interesting stuff ( hint: Mecha-Mouser and Dryclyde might not be totally off the table after-all ), and i'm open ears for other things I could do to make this hack interesting as long as there's someone with the ability to make it happen.
Because of this, I think a new name is necessary, it's not just about artwork accuracy anymore and I think advertising it as such might send the wrong message.
So we have SMAS: Deluxe... just a simple and very common suffix to add to something to imply an upgrade ( though I feel a lot of people might not see what I've done as an upgrade. )
35th anniversary edition, since SMB series will turn 35 this year. The franchise as a whole will be 40 next year, but I don't see that hack taking this long ( although, if someone could insert Classic Kong into the game, I'd feel obligated to rename it 40'th anniversary instead. Not likely. )
SMAS Redrawn, gets the point across pretty well but a little derivative of SMW Redrawn, which is pretty different from what i'm trying to acheive here and will be even more confusing when I get around to the SMAS+W version of the game.
SMAS 2020 ...pretty obvious. That's the year this Hack will hopefully be finished in.
And lastly, SMAS, classic edition, which is mostly in reference to the way this hack is trying to re-create the classic look of the games as well as the classic artwork ( as opposed to the modern, NSMB style stuff ), but also just a nod to the SNES classic edition, which I really want to put this game on once i'm finished.