A Flint or Duster AT is a possibility. I wouldn't bet on a new Mother series rep, though. If it happened, my bet would be on Porky/Master Porky.
No, no, I understand and I wasn't betting that Nintendo would put a new MOTHER series character in for SSB-Ultimate.
Just thought that the lack of either of them in Lucas' Final Smash, could have been a possible clever/hidden hint about either of them could be used as an Assist Trophy, or they could have made one of them to have joined in on as a playable character.
My reasoning is because of their very unique skills and talents, not just in MOTHER 3 or the MOTHER series in general, but in Nintendo's roster as a whole.
Ofcourse, Master Porky falls in the same league too, so I do understand that.
But while there is a chance that Nintendo might put them in, we'll have to wait and see. As like you said, I won't have unbelievable expectations that either Flint or Duster will join, or that Nintendo will add a new MOTHER character.