I don't know about improvements. Would you want to just make them equivalent to the regular improvements, or come up with something else entirely? If so then walls, barracks, harbors, granaries, and others could probably stay the same.
I guess there are few ways to do technologies and ages. The big problem is that Middle-Earth technology seems pretty much to stay the same or even degrade a little as the ages go on.
Would the game just cover the second and third ages? Or would it start in the first age and go to the fourth age? In this case probably some fourth age techs could be made up. I'd have to really think of some techs, these three are about all I can think of right now:
Ringmaking (enables Great Ring great wonder and Rings of Power small wonder)
Ride of Orome (horses appear on the map)
Tengwar (same as alphabet)
As a replacement for jungle, how about Wastland or Lava Plain? It could have a movement cost of 2, no food even with irrigation, 3 shields (4 if mined), and maybe cause disease? Sort of like a floodplain, only with shields and not food. That way on a map of middle-earth it could be used for Gorgoroth terrain. Another replacement could be swamp, with one food and no shields, and causes disease. IMO Lava Plain would be more useful, but swamps are more widespread in middle-earth. Oh, and I like Snoopy's terrain too.
What should the barbarians be? I don't think any smart race, maybe spiders and other monsters?
Also is there any way to change culture groups, or even add more?