So based on what we know about characters that AREN'T coming as fighter #5, here are my theories for other characters we WON'T get:
-Anymore classic arcade fighting characters. Firstly, Terry was JUST released as #4. Having #5 as another arcade fighting character would be totally against Sakurai's MO of creating diverse movesets, especially so close together in the same fighter's pass. MAYBE in the next pass, but I'm 100% we won't see one as fighter #5.
-Hyper obscure pick. I know that many people think that pretty much ANYTHING can get in at this point, but let's me honest here, the DLC thus far is trying to appeal to pre-existing fanbases. I highly doubt that fighter #5 will be a complete "literal who?". It's going to be someone that's highly recognizable even if those that recognize them are outside the smash fanbase (and likely will be).
-Hyper hype pick. As fatman and others have pointed out, Nintendo is legendary for tripping at the finish line and landing face-first into a pile of swirly dog poo. That said, I highly doubt that fighter #5 will be anyone from the top 15 or so of any smash ballot we've seen. I also doubt it will be a character that generates seizures of joy a la Master Chief. Speaking of which....
-I highly doubt Master Chief for two reasons: Firstly, for the reason stated above. Secondly, because of his similarities to Doomslayer (both aesthetic and potential moveset-wise) I feel that his inclusion would be almost as good as a deconfirm for Doomslayer, and by extension, the Cacomallow leak. The leak that not only signals Geno but also the one that has yet to be disproven as arguably the most convincing leak of this speculation cycle. Frankly there's a dozen other reasons why I don't think he'd happen, but those are the largest two to me.
-Given the pattern of DLC thus far, I highly doubt we'll see a character from a company we've seen represented so far. So that leaves us with what? Bamco, Ubisoft? Suda 51?
With all this criteria in mind, the only three nameable characters I can think of are Arle, Travis Touchdown, and Rayman. Each is renown in their own right, but also didn't top the smash ballots and comes from a company yet to be represented in the DLC. Each would also offer a fun and unique moveset which we know is a priority to Sakurai. If anyone has any other ideas that fit these criteria, please let me know.
EDIT: I forgot about KOS-MOS. She not only fits the bill perfectly, but would also add some of that female diversity to the DLC roster. Not to say that we HAVE to have diversity, but I think a female character of her caliber would definitely stand out from the rest.