The first thing that I would say is that I think the stronghold should be approached like it is a character unto itself. What I mean is that I want to "level" and customize my stronghold similar to what I do with my own character. I enjoyed the Cross-Roads Keep in NVWN2, but thought the mechanic was under developed. Here is what I think would be the optimal utilization.
Starting location: Instead of one stronghold location I think there should be 4 possible options when the stronghold is introduced in the game. You choose the one you like and the other 3 locations do their own thing throughout the rest of the game influenced by the players actions. That way the other 3 locations aren't wasted assets, and can serve as quest locations. Each location could be altered to be more commercial, industrial, military, etc. Here are some examples of various starting locations:
Country Side - This could be a more peaceful option that is more suitable for growing crafting resources and watching over peasant farmers. Maybe the player would treat their stronghold like a land baron presiding over a vast expanse of subsistence farmers.
Trade Route -This stronghold would be on a vital trading artery requiring protection and creating obligations on the player to deal with the merchants passing through. A rogue could customize it to be a thieves' den, a mage could customize it for trading alchemical ingredients/mystical know how, a fighter could make it a training ground to train better patrol guards, a crafter could focus it on making it better for crafting then trading items with the passing merchants.
Sea Side Port - Depending on how much seafaring Obsidian wants to introduce this could be a fun option. The stronghold could be customized to be more like a pirate den, protecting the high seas, or focus just on trade.
Area of Conflict - This would be a stronghold that requires the most management by the player because it is in a strategic military area. The player would have to focus on martialing troops and improving defenses. One fun element could be that the stronghold could be captured for failing to improve it enough so that the player has to recapture it.
The takeaway is that it don't think the selection should be here is commercial, here is industrial, and here is a military location. The locations should have unique feature then the players could customize it towards what they want.
The final suggestion I have is that the stronghold should grow in a more organic way. Instead of paying X resources for an add on and then it magically appears after purchasing it. I would like to see a mechanic were all improvements need to be funded then built overtime. The length of time would be based on availability of resources, number of workers, and their skill level. So if a player wants to build a massive stronghold now, they have to find people to build it and resources to accomplish that goal.