1. India
India has not received any real attention since Art of War, and has a long and fairly well recorded history. It is also one of the few regions that decidedly lacks its own "flavor mechanic" (read as: HRE, Colonial Nation, Emperor of China) and could use a unique mechanic to spice up the area. It also needs to be designed in a way that the Europeans can serve as a late game shake-up to the local balance of power. The DLC could include mechanics both for the Indians, more religions in the area, and mechanics for Europeans and their Indian holdings.
2. HRE
HRE can always have more countries, and could always use more depth mechanically speaking to better represent how major and central it was to European politics. Austria also needs a face lift and would be a suitable candidate for a unique, diplomacy centered government form.
3. Western Europe
Most of Western Europe could use a face lift; it hasn't been touched in quite some time and France, Spain, and the Low Countries are in need to a face life to better reflect how they were at the time with the same care that the Ottomans and the Middle East have.
4. The Rest
The rest of the regions you listed could certainly use map updates, with more detail, nations, and ideas. But mechanically speaking I don't know the areas well enough to propose or think of a reason for their own unique mechanics warranting their own DLCs. I would love to see Africa get a unique mechanic (or several) if there was a historically plausible system that could be modeled, and I would like to see New World gameplay improved for both the Natives and the CNs, but I cannot think of anything major enough to require a dedicated DLC.
