OK, so going by SSB history, at least one series got 2 Unique fighters in every installment. Pikachu and Jigglypuff for '64; Bowser and Peach for SSBM; Lucario, Pokemon Trainer, Metaknight and Dedede for SSBB; with Rosalina and Bowser Jr for SSB4. We also got series with one at least unique fighter and at least one clone: Luigi ('64); Roy, Ganondorf, Young Link, Falco, Pichu and Doc added to FE, LoZ, SF, Pokemon, and Mario (Melee); Toon Link, Wolf and Lucas joined the triforce, space animals and Ness (Brawl, but Lucas and Wolf are barely clones) while Pokemon and Kirby get 2 brand new fighters per series. SSB4 gives us Dark Pit, Lucina and Doc while Mario gets 2 new fighters, Kid Icarus gets Palutena and Fire Emblem gets Robin and eventually, Corrin.
Basically, what I'm getting at is that there's a trend where some series get 2 unique characters and others get a unique character and a clone. So far we've seen Metroid and Castlevania get the latter and since y'all know where I'm coming from, there's a chance DK can get the former. Only thing that stands in this idea's way is the very real possibility of some other series getting this. Barring DLC in Smash 4, the only time 2 series got 2 unique characters was Brawl in the form of Pokemon and Kirby. We know for a fact that Ultimate won't have that many characters, so the Brawl scenario's not happening, and Echoes are a good solution to this.
What we don't know is if Ultimate will be the 1st in which no series gets 2 Uniques. I think there's room for this not to happen, mainly because of the massive deconfirmations of characters from popular franchises and DK roster-wise is small enough to get this, but so are other series. Luckily DK has popularity going for it, a clear focus on content finally and not many options that rival Dixie's significance. DK COULD get lucky and have 2 Unique fighters. It's not a guarantee but it's definitely a possibility.