While I see 6 as a plausible number for newcomers, I wouldn't expect all of them to be completely unique, so I'm just going to go with this arrangement:
1) Isaac (Golden Sun)
Isaac just makes a ton of sense to be the next big character, he's a crowd pleaser in terms of character requests, he has moveset potential, he reps a very underrepresented series and his inclusion and the newfound exposure for Golden Sun could lead to more confidence in Nintendo and/or Camelot to revive the series.
Needless to say, he would be the biggest remaining no-brainer inclusion, if it were up to me.
2) Rex & Pyra (Xenoblade Chronicles)
That is of course if I, in this dream scenario had enough insight in XC2's development to have enough material for them to work as Smash fighters.
I just love XC2, so this is largely personal bias, but the success of the game and Xenoblade's steady rise as a franchise would be more then enough justification to have both them and Elma in the game and that's not even going into their moveset potential.
3) Arle Nadja (Puyo Puyo)
Personal bias, but since the west already got Simon as it's highly requested third party character, she would make for an excellent Japan counterpart. It helps that she does have a western following and the localization of Puyo Puyo Tetris did it's part to give the series a larger western presence then ever before.
4) Kat & Ana (Wario) (Ice Climbers semi-clones)
Wario is overdue for more content in general and the Ninja Twins have even had a presence in Smash before, It's possible that they might not quite work as straight up Echos, but I can easily see them work as semi-clones, with the neutral and down specials, as well as their Final Smash, changed or adjusted in a way that fits them and the hammer based attacks getting converted to sword based ones, that still work in a similar enough fashion.
5) Ninten (Mother) (Ness Echo)
An easy Echo to make, he rounds out the Mother representation very nicely and plenty people like the idea, so why not?
6) Shadow (Sonic the Hedgehog) (Sonic Echo)
I'm willing to bet that he's in Ultimate as is, because he is just one of the most natural and obvious Echo choices.