impatient bump 

Actually you could take this idea even further - to the point where resources could not only be worth defending but vitally important -

Here's my idea - you can build on a resource:

a- the original building (bonk)

b- 2 upgrades that increase output/efficiency

c- after productivity is maxed you can build 1 of 2 possible 'end upgrades' for the resource.

eg. An old growth forset (as Kingdom)

1 - Lumber camp

2- Lumber camp lvl 2 - +1 mat

3- Lumber camp lvl 3 - +2 mat

now one of the following:

a ) Centaur's glade - allows connected city to train centaurs

b ) Dryad Shrine - recruits a powerful forest dryad as a champion - respawns when killed, dies if shrine is destroyed

Not only does this place a greater (eventual) value on protecting your realm as opposed to garrisoning your cities, it also allows for greater diversity of units on both sides instead of loads of men in gear. I was a tad annoyed when i found that the only recriutable creatures on the kingdom side were misunderstood monsters.

Anyway there's my idea.

I think it would create a few interesting viable strategic options, here are a few off the top of my head:

a) City Spam - resources used to produce many weak, low production outposts, relying on crowding out other factions.

b ) Balanced Growth - expand until at least one of each resource captured, then invest heavily in upgrading your resources.

c) Blitzkrieg - invest little in resource tech, rely on others to do so, then roll in and steal their upgraded resources.

d) Harrassment - invest in a small mobile force that can raid behind enemy lines, taking out key resources, forcing enemy to leave their cities.

e) Scorched Earth - destroying all resources prior to a losing a city 

I'd see the investment required for the fully upgraded resources to be quite high - in material and manpower (they would have to be staffed, after all...)

No GalCiv constructors, I agree that these were far too time and effort intense. Just a large amount of material (which could have been used for buildings, armies etc.) and a commitment to a certain tech branch (which could have been used to research more spells or better weapons).

These games are all about taking difficult choices, and the ability to supercharge your resource base must come at the cost of being unable to take other choices (the payoff coming once the produce starts rolling in).

In a way, it already exists.

Since most buildings give percentage bonuses instead of static points, it is actually doing exactly what you are describing: Research technologies to improve the productivity of a resource.

The current system also has advantages over your suggestion: It requires you to upgrade your city to higher levels to build some of the improvements, and requires you to spend space and resources to upgrade.