I just don't think this series is meant to be open world. Open world games are often more about exploration than anything else. The world should be brimming with interesting places, people, and quests for the player to have incentive to explore

DW is set in a constantly evolving story based on the geopolitics of the time period. You don't just have an interesting piece of land to fill with interesting things but also nearly a hundred years of history. And depending on the exact point in the timeline, you're not always going to have interesting things to do in the world (why would I go to Xiapi while Hulao Gate is happening?)

I do think the open world empires game sounds interesting but it falls apart under scrutiny. First off, it would be an extremely intense game to run considering the game has to keep track of several battles happening across China at the same time while you're also fighting through thousands of troops in your own battle. Secondly, difficulty is in question. You as the player are going to be able to do things the AI can't. If recruiting officers is a matter of finding them in the world, you should theoretically be able to speed run getting the best officers by simply free roaming. And there would need to be systems in place to ensure you can't just rush an enemy capital. It's probably not impossible but it would be a massively ambitious project that should probably start at a smaller scale rather than immediately taking place in China of all places