I am starting to think more and more about what stages these characters bring and not just the characters. I mean, one f the best stages in Smash Wii U/Ultimate (U/U) is Midgard from FF7 IMO. And with new characters and new stages we get new music also. I have only played the first Halo on the original Xbox, so I can not tink of any good stages or songs from taht being added, but that's my lack of interest for the IP.
Still wishing for TErra from FF6 (come on! just imagine what music could be added... But Narshe as a stage?) and Travis from NMH. Also a great character that would fit perfectly in Smash and at least we would get more awesome music.
I don't think that all of the DLC characters will be 3rd party though. I am sure at least one or two will be from Nintendo's past but e might be as surprised as with Piranha Plant (who turned out not just to be a great fighter, but Sakurai and co truly made him damned charming and full of life).
But as Nintendo and MS are flirting so much these days, I wouldn't be surprised to see Metroid X Halo one day.