I think it‘s safe to assume the battle mechanics are going to be similar to how they were teased with Reimu and Mokou. In ULiL extra, only Reimu's body took part in the battle, with Mokou's body still being somewhere in the bamboo forest. But at some point Mokou's soul(?) took over Reimu's body. After the fight, Reimu regained control over her body, but she still suffered from the burns Mokou gave ‘herself’ during the fight. In other words, Reimu's body took Mokou's ‘damage’.
So, a 2 vs. 2 fighting system or an assist system would seem very unlikely since each team has only access to one body during the battle. I expect the fighting teams in AoCF to be able to switch fighters mid battle, but they will use the same life bar since they use the same body.
That's sound plausible. But then again, the game will focused on pair, similar to Imperishable Night; a team with two individuals with different personalities must work together to solve the Perfect Possession incident. No matter if they like it or not, voluntarily or involuntarily. Each pair have their own method to solve the incident.
Speaking of that, I re-read the premise of the game's story. But I found the most interesting thing about this part: "those who exploit it, those unwittingly dragged into it, and those working to expose the threat behind it..."
Let's say in AoCF, the complete cast of ULiL re-appear in this installment, along with some unexpected characters and newcomers alike. However, because the story itself sets between LoLK and the yet to be announced 16th main game, we can assume some LoLK character will join the fray. Here's some characters who keep their eyes open on this Perfect Possession:
- "Those who exploit it":
Some participants who involved in Perfect Possession incident just for curiosity, to increase their fame, to fulfill their own goals, or just for the hell of it; such as Byakuren, Miko, Nitori, Koishi, Mamizou, Shinmyoumaru, Junko, Hecatia, and of course the main perpetrator(s) of the incident.
- "Those unwittingly dragged into it":
Some participants who remain neutral from this matter, but unfortunately they forced to be involved in the incident; such as Ichirin, Futo, Kokoro, Mokou, Reisen, and possibly Sagume, along with Seiran, Ringo or even Clownpiece.
- "Those working to expose the threat behind it":
As per tradition, it's Reimu and/or Marisa. Apart from that, we can expect other incident resolvers (AKA Stage 5 boss turned playable character in main games) will join the fray like Sanae, Sakuya or Youmu. Others who want to resolve this incident are Kasen and possibly Sumireko, the latter is trying to make up for what she's done in ULiL.
In regards of LoLK casts join into AoCF:
- If Sagume joins the roster, then we can expect the story will focused on how she tries to clean up the mess she done during the events of ULiL and LoLK. However, as andykhang said, ZUN doesn't like putting an overpowered foreinger into the roster. Instead, she will entrust her duties to Seiran and Ringo on her stead. But then again, we can only wait for a confirmation from ZUN.
- If Junko and/or Hecatia join the roster, we can expect the reason why they involved in Perfect Possession incident just for continuing their plan to avenge their respective relative's death towards Chang'e.