I'd like the 'state of the galaxy' to be a little clearer in VIII, and fewer echoes in story structure of ESB than TFA had of ANH (seemingly both common sentiments)
Any answer would be grounded in basically nothing. Pointless speculation. I was more excited by the announcement of Johnson, FWIW.
All have potential, but Poe I guess. I read Before the Awakening recently which gave him a bit more characterization/backstory. I'd like to see him have more of an arc in VIII. Seeing the dynamics of a Rey/Finn/Poe trio would also be interesting.
TFA did a fantastic job of setting Kylo up to go any of several directions. I see him as further entrenched in denial of the light side, trying his utmost to convince himself he must be truly of the dark side now that he's killed Han, but still struggling internally. That might manifest as seeming more focused externally in general (i.e. less prone to public tantrums) but actually being even more of a powder keg when it comes down to vengeance against Rey/the Resistance.
I assume the Finn/Rey friendship will be a significant aspect of further films. They are each other's first real connection as adults, and clearly care for each other. My guess is there won't be more than hints/banter with regard to any romance for Rey.
I'm not starving for more of either, but certainly expect to see both.
If a major character had to return, I'd be fine with force ghosts of Yoda, Obi-Wan (McGregor, naturally), or Anakin. For a minor character cameo, Ponda Baba (now lacking both arms).
I'd prefer more hints in VIII.
AFAIK, there's little indication of the First Order's capability or territory beyond Starkiller Base. It'll be hard to tell whether they've expanded without being outright told. I assume they will have rallied from the destruction of Starkiller, though it might be a refreshing twist to see them on the run from the Resistance/Republic, Rebellion-style, at the start of VIII. After the destruction of Hosnian, it would make sense that what's left of the Republic's government would no longer resist military conflict with the First Order.
There's certainly a chance, by definition. She has an edge, a history of being unable to "let go," and the novelization of TFA made it explicit she was tempted by the dark side during her duel with Kylo. I think the odds are very low she'd turn dark and stay there. She'll probably tap into it occasionally and struggle with overcoming it like Luke did.
I highly doubt he'll be Sith, and would be mildly annoyed if he was. If I had to guess, he's a former Imperial officer either vying for control of the First Order or leading another ex-Imperial splinter group.