I think if being on the level of Street Fighter is a requirement for a character to be a DLC newcomer, then we are not getting any DLC newcomers besides Ryu. We could get Snake back (veteran) and that is it. Even the requested by some Simon Belmont isn't on the same level as Ryu, I think. Who else there is?
I think it is also important to have in mind that what is considered a niche title in the west is not necessarily a niche title in Japan. Rhythm Heaven is as niche as it gets here, but quite popular on Japan (according to popularity polls, at least). So while I agree extremely obscure characters have no chance of being DLC (like say, Lip from Panel de Pon or Takamaru), characters like a Rhythm Heaven, Agent 3/Inkling and maybe Wonder Red shouldn't be discarded yet.