Thank you for your suggestions!
At this point (after some minor tweaks) I can start working on World 5. And this is the first progress I have made:https://youtu.be/tfc1ssvdujc
I think the changes integrate well with the original graphics and fulfill what someone who had played Chinese Bootleg when they were little would have expected (I've also added a little joke, just for fun
And now, a few questions:
This cute UFO is begging to be changed. Someone on my Youtube channel has suggested Lakitu (which could cause problems, because if I remember correctly it measures 24x16 or 32x16 with the cloud).
About the pig, it is clear to me that the most obvious choice is a Boomerang Bro. And the wooden soldier I will try to change it for the tin soldier from SML2.
And this tiny robot... oh, how I hate it XD. Seriously, it looks really uninspired. It shares palette with the UFO and has an annoying attack routine: When you hit it it gets damaged, but it keeps moving, which causes if you're too close it causes damage to you.
And of course the boss. Apparently the development team had a certain fixation with giving the Final Bosses a somewhat creepy and menacing look, with spikes and icy eyes.
Plan A is to alter the design to make it more "friendly", Mario style. It's a good solution, because in the end it's a game that arouses nostalgia, and changing it excessively can be detrimental... unless you come up with something that fits like a glove instead.
And that's all for now. Nothing is closed yet, so, let the Brainstorming begin! And thanks in advance for your help.