I have a feeling that the next fighters will be the most fan-requested ones, here is my theory on why :
First of all, if this direct convinced me of one thing, it is that the smash devs kinda had to add Byleth because Nintendo wanted it.
They knew adding Byleth would not please the fans as proven by the "too many FE sword users" joke, the fact that Sakurai said he hoped we wouldn't be too mad on top adding a sixth character in the second pass to make up for it (imo).
Knowing that, it is possible to assume that the Smash devs are well aware of what's going on within the Smash community ( Sakurai even mentionned the twitter suggestions ) and that their efforts will be turned towards pleasing the fans.
And guess who has been among the most requested characters all along ?