In an early version, there was a resource called "Mages' Tower". It allowed you to recruit Healer Mages and Elementalists. If I recall correctly, the reason I nuked it was that the Monsters & Mercenaries faction has only 2 Standard units; almost everything is Core or Elite. So the Healer Mages being a tasty Standard unit means that the Market is a useful building for some factions (the Horse Archers are also useful to some factions).
There are obviously ways you can divide up the tech tree other than the arbitrary core/standard/elite that I did. The buildings you propose use the model of "building matches particular type of unit". So to make the whole tech tree like that, you would want a Stables to allow for mounted units; an Archery Range to allow missile units; and so on.
I think that I might like this system better; certainly, it tells a better story (why does having a Market let you buy Standard Mercenaries, anyway?). But it's wicked hard to balance: buildings that are essential for some factions are useless for others. Some factions would be able to recruit all their units with just a few resources, while others would need to build many. I'm sure it can be done, but it's not simple or fast.
With regard to the last 3-- the magic armor, the illusionary unit, and the +2 command actions-- this opens up a whole new can of worms. (Not that this is a bad thing!) One decision I made relatively early in the design process was that no Kingdom resource would effect a Battleground game once the game started. I did this for a number of reasons, but the two main ones are: (1) it would be Yet Another variable in our playtesting of the scenarios/maps, which was already hard enough; and perhaps more importantly, (2) I wanted the focus to be on playing Battleground, not on using Kingdoms to gain abilities that changed the normal flow of the game.
This decision really made putting together the tech tree pretty hard; there's only so many meaningful abilities you can give someone that only effect army construction and deployment. If I had allowed myself magic armor and weapons, and officers that gave all sorts of bonuses to command and rallying, and so on, the tech tree would be larger and more varied.
But, one of the reasons I felt so comfortable leaving the tech tree so basic was that it's so trivial to change it! It's not printed on any cards; anyone that wants to can come up with an entirely new one, or modify the existing one. So my intention was that if Kingdoms catches on, and generates any reasonable amount of online chatter, we could collectively come up with some alternate tech trees. So I'm glad you're already thinking along those lines.
(And BTW, I am currently really excited about a completely new set of rules that I'm working on even as I type this, that uses the Kingdoms cards but not its rules. It's a completely new direction, and even futher away from the standard idea of a campaign system than Kingdoms is. I'll post it as a free .pdf when it's done, assuming it's fun in playtesting; then you'll have 2 games for the price of one.)
