Here are some ripped and custom sprites. I hope Jay makes similar sprites for holding the Hammer. http://www.mfgg.net/index.php?act=resdb¶m=02&c=1&id=21885
Don't hate me for this, but I say just make your own levels. If you have to modify the levels to fit the characters anyway, why not just start from scratch? It'd be awesome for SMC to have it's own expansion!
That should be low priority. I say go like this: SMBS LL BROS SMB3 (maybe) Entirely custom levels You could also maybe do DK'94, but again, low priority.
I thought we could just make minor modifications to Donkey Kong's hammer sprites. All Night Nippon is just a graphics hack of SMB1.
VS and ANN didn't really add any new levels, did they? They just had a blend of SMB and LL levels, with some minor layout changes. Not worthy of separate inclusion. Edit: According to Mario Wiki, they removed the wind effect, changed some block placements, and changed some castle maze solutions in ANN. Make those toggled options, if anything.
SMB3? That's 92 levels! It's a lot of work! Plus ramps and new powerups. Would take a good two years to get SMB3.
Great Giana Sisters is a completely different game. Yes, it's SMB style, but it doesn't need to be included in SMBC as an official level set. What is BROS? As ReNeX said, this is a lot of work. I know you said "maybe" and it's low on the list, but still, I really don't think it'll happen as a normal level set for SMBC. The levels are quite a bit different from those in SMB. The SMB3 levels would essentially need to be a whole new game because of all the different stage elements and power-ups. I do think a SMB3 Crossover would be cool (see Mario & Sonic at The Mushroom Kingdom), but the levels themselves simply wouldn't translate very well into the SMBC engine.
I just reallized: Mario Land levels would also be pretty easy to implement. The Super Ball could probably be safely replaced by Fire Flowers without too much issue.
I thought Giana was a clone like BROS. If not, then there's no need to include it. My thought on the Hammer is that it would be the Mario Bros answer to an armor piercing weapon.
IT IS STILL AMAZING. Nah, I'm joking. It's basically levels from SMB1 and LL with blocks and Hadoukens, anyway.