I think
Age of Calamity might be better off keeping it simple in relation to the Warriors formula, which means trying to capture the open world of its sequel might be too ambitious an undertaking. DW9, as much as I respect its ambition, did not quite hit the mark; and if mainline
Musou games have not handled the concept of the open world with the beauty and grace only befitting Zhang He, then it might not be a good idea for this game.
3. Open World. Think Dynasty Warriors 9 but actually done well. This would just be a small number of really large maps. Kind of like how BotW is. But you'd have the musou combat scheme and potentially have mini battles in certain areas. Scriped story events within the world.
I definitely find myself skeptical of the open world concept in light of DW9, but I think having large battlefields for each scenario can work. BOTW has enough diverse landscapes to accommodate numerous battlefields. I can see Hebra's mountains perhaps functioning in a manner like
Dynasty Warriors and its Hanzhong missions like Mt. Dingjun. Perhaps even some naval battles could be interesting (although I am not certain there is a large enough body of water to accommodate).
I do like you note about mini battles in specific areas. It reminds me of how
Dynasty Warriors 4 split missions for specific areas. For example, one could choose to play Shi Sui Gate, a smaller battle, before delving into the gigantic mass of Hu Lao Gate. The idea is certainly interesting.
I would totally love a side mode where you can free roam around maps with no objectives other than to beat the stuffing out of as many enemies as you want. Maybe as a means of grinding too but also just for the fun of it. I could spend hours doing that. Even better if they gave us BotW's actual whole world pre-calamity to do this in. But that might be asking for too much.
I think that might be interesting, although I wonder if the game will need to populate areas heavily with enemies in a way similar to
Dynasty Warriors's rampage mode where you have to slay as many enemies as possible until you die? Perhaps the game can have a larger area of scope?
BotW's world is really open too, so I'm think that a lot of levels are going to be rather large, which would fit into the freedom BotW gave players.
This is pretty much my hope, as I think large level sizes can work without necessarily delving into open-world territory.
______________________________________________
Now, pulling back to the OP, I only have one tweak to make to the already-established formula from the first game, and that is not having dedicated weak points for normal officers. I did not mind it for the bosses because it was a staple of the main series and worked decently enough. As for regular officers, I found it broke the pacing of fights because instead of actively combating the target, one dodges and dodges until a weak point is exposed for large damage. This I find to be detrimental, especially for weaker-leveled officers where defeating just one officer could take 5 to 6 minutes on average. If this were the exception, I would not mind it as much. Otherwise, I think
Age of Calamity is in a good place with what its predecessor started.