Lucina's really significant to IS/Fire Emblem, and with her being planned to appear in new FE games, she probably won't be cut. Maybe turned into a semi-clone (similarities but still very unique and large differences).
I don't see us losing both Greninja and Lucario, maybe one of them or neither.
Honestly I don't think we'll be losing as much for the next game; at this point in the franchise, SSB4 includes almost every notable Nintendo character. It wouldn't make sense to cut out some characters for SSB5. It made sense to cut melee characters in brawl and brawl characters in ssb4, as they can just be added back on later and their return would be more exciting than simply being in every game since they were introduced. I imagine SSB5 (if it is going to even prioritize adding a lot of characters as its main selling point) to have the same roster except maybe 2-3 cuts (like dark pit and dr mario), the return of cut characters like Lucas, Wolf, ICs, a few missing fan favorites from SSB4, a couple less popular but still notable side characters from major Nintendo titles, a new retro/joke character, and a couple characters from new franchises (2013 and beyond), totaling about 12.
I think it's also possible SSB5 may have very few newcomers (like 5, and a few returning vets) and have some new focus. Brawl had a story, SSB4 has customization and better balance. SSB5, if it will even come out, might go back to having a really awesome campaign mode. Or SSB5 will still add a lot of characters and then SSB6, if it is ever made, will focus on other things.
Also please don't call it SSB6. If you consider Wii U to be SSB5 then call it SSB7, because there is a "SSB6" in 2015.