I haven't been following their Ci-en that closely, but I remember seeing a relatively recent post that seemed to indicate that they'll use the same system as the last two games in terms of play-style... so the second item on your list is likely a no-go. The post was also talking about the fact that the story they wanted to tell seems to require too many characters, and that in this game format, that means more character sprites and stuff and that they weren't sure if it could be manageable. I may have gotten it wrong, but it sounded like they were considering massively simplifying their plans.
Either way, VERY low odds of there being more than one protagonist, whoever it is. Maybe one more temp protagonist like in this game, but I don't know if I'd hold my breath for that either. It didn't sound like the dev was that fond of the system and its results.
Then there's the fact that parties and battle-fuck systems don't mix well. The circles that do implement party-battle-fucks have to downsize the CGs when multiple party members get fucked at the same time and often have to simplify the system in various ways because of the multiple party-members. That's not to mention the failures of some of the systems, or the poor implementations that can slow combat to a crawl as multiple girls go through multiple sex mini-scenes in the middle of combat... over and over again with little variation.
It'd be hard to implement for Hasoyua, because they have a more complex battle-fuck system that involves a lust gauge, effects on health and MP, clothing damage and sensitivity levels. Multiple party members means doubling all that for 2, tripling for 3, etc. It could very easily turn into a mess.
Guess we'll see, but I don't have very high hopes tbh. I think the dev kind of peaked with Seraphina's game and his attempts to top himself there haven't been that well thought out.