Well, as I already said myself, it's probable that this will be how things end up fleshing out, because I just don't think PS are preparing for a season three and the core staff is going to have to be shifted away from continuing support for this game in favour of other projects, eventually. So to the extent that there's realistically only going to be so many more characters added in the projected second season pass, I agree it's almost certain that they will favour the classic characters over anyone introduced post-SCIII--and that they have given every hint that this is exactly what they are doing. But don't kid yourself, if they had the free staff and were finding the serial content for SCVI lucrative, they wouldn't hesitate for a second to bring back remaining popular characters just because of some thematic constraint. I mean, if it came to that point, I'm sure they would happily just expand the period of time covered by the events of SCVI. Alas, I'd be surprised if even their zealous approach to the post-release support of this game will go as far as a Season Three. One can hope however.
I think you'd better be prepared for disappointment then, because I'd say Rock is a virtual lock, given the evidence we have. Kind of a surprising turn of events given how portions of his moveset were rolled into Astaroth, I agree. But given how much this team is enjoying monkeying around with the classic styles (to....mixed results, in my impression) I think they could easily make it work. There's no reason they can't, as a stylistic matter, give him his mace/massive stone warhammer. I think Aeon is likely to end up switching from his sword to the axe earlier than he did in the original narrative, based on what we have seen of the Lesser Lizardman style, so I don't think they are following strict fidelity to the original run of games in that respect. I think he'll be fine, in the final analysis. He still has the vast majority of his moves, even if we assume the ones given to Asta are his alone now. As an example, they made shit-tons of new animations for Amy--most of which (minus the flowers) are pretty faithful to her general style. I mean, the truth is, Rock was so low tier in SCIV that you would have to really mess up to make his fans more disappointed this time around!