You have my undivided attention. Looks like a very interesting idea.
I would think that outposts should have fewer level ups....maybe only 3 instead of 5.
I would also change your naming convension: 'Hamlets', 'Keeps', and 'Wizard Towers'
May I recommend that level 1 outposts all share some basic upgrade options. Maybe 3...
Then when you level up into your specialization, you get acces to 5 more of that area.
The third level up then provides additonal bonus, aside from the upgrades, and specific to the type of outpost it is.
Really like that the 'upgrades' from the original outpost concept are spread around through the level 2 upgrades.
Don't know how the outposts would level up though, as they do not have a growth. Perhaps, it can be based off the number of upgrades it has.
2 upgrades, and the outpost goes to level 2.
5 upgrades, and the outpost reaches level 3.
You may still of course build more upgrades for their bonuses.
Another thing to consider is that if these bonuses are too powerful, then outpost spamming will result. There must be deterent, such that the user only wants to build outposts where he'll gain resource, magical, or military benefit.
This is an idea that I hope Stardock really strongly considers.
Great idea Satrhan!!
As a side note, I really think your capital should be able to reach a level 6 with additional bonuses and perhaps a building that starts some end game quest or spells to help win the game.