If you're in for suggestions, I would say check the list of games featuring Mario for the NES:
I'd start by developing a small Luigi sprite first (since it could be used for the largest number of games) and working my way from simplest to more complex. Clippit's Luigi Bros looks good, but I don't know if he edited the sprite at all or just did palette swaps. If you're so inclined, putting a tie on Donkey Kong and possibly even replacing DK jr with Diddy Kong could help to modernize it as well.
Clippit's Luigi Bros. is a direct copy of Nintendo's Luigi Bros. as included in Super Mario 3D World. The only changes are the title screen, palette-swapping Mario to green/blue Luigi, and Luigi to white/green Luigi (so it's Luigi and Luigi). You can see Nintendo's in action here: https://www.youtube.com/watch?v=Ds0X4QkfeC4
That's kind of the style I want to go for, too. Nothing super fancy and upgraded, but subtly removing Mario and inserting Luigi in his place. I'm not really the artistic type, so honestly I'd rather leave things like that to those inclined. I just wanna make a simple set of hacks that put Luigi in the spotlight, like Nintendo's taken to doing. (They've done another instance of this in NES Remix, where some Donkey Kong stages are played as Mario palette-swapped into Luigi.)
That isn't to say that every hack is just going to be a palette swap; some of the later games had more fancy graphics and smaller things that could be changed for fun. Dr. Luigi is a good example; I couldn't just palette-swap Mario since he's just wearing white in the first place. I had to redraw him as the taller, slimmer Luigi. It's still Dr. Mario as you mostly remember it, but with the other brother at the helm with his new viruses. Dr. Mario also had these really unique but weird intermission cutscenes that I'm currently modifying into fun little different things. That's the part that's holding up the release; I've gotta decide what to put there but I also wanna keep it a secret so people are pleasantly surprised when they beat the required stages.
But yes, I actually have made a list of all the Luigi-ization candidates I'd like to do if I can find the time. Guess I'll post it now:
The names on the left are the originals while the names on the right are the Luigi versions. Anything filed under "maybe" is something that's either really obscure, kinda already features Luigi as a playable single-player character, or would take too much work to bother with. This is just for fun, after all.EDIT:
(Feels like I make my posts here really long-winded. I apologize for that. Hope it isn't a drag to read.)
Looked through your work, by the way. You are lightyears ahead of anything I'm capable of, artistically. I can see why you keep coming up with all these extra ideas! I don't mean to just outright shoot them down, I just wanna keep on-track with the simplicity I originally had in mind so I don't spend ages redesigning every little thing.
I also noticed you have a Personal Projects thread for your stuff. Maybe after I get some mockups of the other games, should I make a new thread there for "Year of Luigi Series" discussion? I'd think that would attract more discussion from newcomers rather than my current OP with dusty old pictures of an outdated version of Dr. Luigi.