I'll try to adapt to the environment and build what I believe best suits the local people. What will happen most likely though is I'll end up mostly building fortresses to support trade and to support military action. After all you need money to supply an army and a smaller army with superior equipment may very well be more powerful then a larger one that's struggling for supplies. I know intelligence is important but often my mind simply doesn't work that way.
It would certainly be interesting to see if the smaller, stronger army worked out better than numbers, if it does make a big difference that would add so much to replayability.
Now if we could devote a few fortresses to research and development I'd most assuredly build a few of those. A location where smiths and soldiers can try to come up with better armor or swords as well as the mages can develop new combat spells.
Agreed, that would be very useful, almost like AC's training of assassins. But hopefully more complex 
I really want to know the extent each type of fortress in terms of impact to our playthroughs. Will picking a military base effect how well your troops can defend a siege force? Will too much of one type have a negative impact on your army? WIll having a dominant amount of one type open and/or close side quests? Its interesting how the concept may work and i hope it isnt bland or only gives the illusion of changing your playthrough.
I hope it makes a difference, BW did say they wanted choices to matter this time around. So here's hoping! At at the very least that Crestwood quest had a number of outcomes; go after the baddies, defend the fort or defend the village, I think? Maybe if you have more soldiers in the area they will be able to defend the fort themselves?
Does it extend the range of the map/neighboring areas you can see? Does it allow you to send assassins for strategic "hits"? Does it allow you to spy on goings-on of nearby enemy factions (and what exactly would that mean in this game)? Can you send spies on missions?
Man, all of those things would be very cool. Gathering intel on your enemy so that taking a fort from them is easier because you know about a hidden entrance or something would be very fun.