I don't think Luigi was ever in danger. My biggest worries were Ness and Lucas, since their franchise isn't as well known. Hopefully, the NA VC release can keep them. Ike might get swapped for another FE character. Lucario is a maybe. Snake and Sonic might need their respective owners to ask for their inclusion, and Snake hasn't really been popping up on Nintendo systems lately.
Ness will stay because he's been in all the previous Smash Bros and his game Mother 2 is still the most popular game in the Mother series, making him the most iconic character from that series. Lucario should be safe as well considering it was just revealed he has a new form in the upcoming Pokemon X/Y. Sonic will also stay since we still have Mario & Sonic at the Olympics games coming and Nintendo is even publishing Sonic Lost Worlds, so they're still on good terms. With Snake, even though Kojima treats Nintendo like ****, since Snake was one of the most popular newcomers in Brawl with one of the most unique movesets, I'd imagine Sakurai wants to keep him and assuming him and Kojima are still friends, I doubt he'll be cut.
The characters I see having the biggest chance of being cut are Lucas, Falco, Ike, and Pokemon Trainers Pokemon. Lucas got into Brawl because Mother 3 was finally released when Brawl was in development and Sakurai had wanted to include him in Melee. But with the upcoming Smash Bros, it's been awhile since Mother 3 was released and with that franchise dead since the creator doesn't want to continue it and was never hugely popular to begin with, I could see Sakurai deciding one Mother character with Ness is enough for that series.
Same thing with Star Fox losing a lot of popularity since Brawl, Sakurai might feel 3 Star Fox characters is too much for that franchise now and remove Falco since his moveset is the most similar to Fox. Ike is almost guaranteed to be cut since Fire Emblem isn't popular enough to get 3 characters and they have to make room for the newest main star Krom. Plus Marth sure isn't being cut since he's the icon of that series and even had his too games remade since Brawl so Ike is the most likely on the chopping block.
With Pokemon Trainer I could see Sakurai replacing his Pokemon with some different generation starters since the first generation Pokemon had way too much representation in Brawl. Out of the 6 Pokemon in Brawl, 5 were from the first generation. That's why I can see starters from different generations replacing them if Sakurai wants to diversify the Pokemon more.