[Ambershee]

Rather uninspired mod idea.

[*] Multiple maps open at the same time (Earth 2150 allowed for the switching between maps, and the transferring of resources and units between)

Impossible, one map at a time only.

[*] "Field based" resources, like the ore and tiberium fields of older games, which can be:

Very much so

[*] ...harvested by vehicles (much like previous C&C games)

You betcha.

[*] ...have buildings placed on them (like a mine structure)

This can be done too (but with limited effectiveness).

[*] 100% customisable units (IE: you can choose what weapon goes on what chassis and with what additional technology... much like the "MechLab", and in-game

Yep, with a bit of clever model part hiding and upgrade stuff - the only thing is that you can only change a locomotor (the way a unit moves) once, and it's permanent. There's also a limit to what you can do with other upgrades too (such as only one true weapons upgrade - but there are other ways around that)

[*] A researches panel, where you can choose what weapons and chassis you want to develop for use in the unit creator and so create units with these weapons

Using the 'sciences' logic, you can do that fairly easily.

[*] Unit transporters (can't be too hard?)

Too damn easy.

[*] an ammunition system, where units with non-energy weapons (like rockets, 'nades, and bullets, will run out and need to be restocked) in E2150 there are arial-reloading units that land on the unit that needs ammo and resets the ammo count

Also very easy ^_^

[*] a shield system, basically, if an energy weapon hits a shielded unit or structure, an EM shield bursts open and absorbs the energy projectile/beam (but this also deplets it)

Sort of, but I doubt you could achieve the exact effect u're after.

[*] I don't suppose you could also mod ZH to allow tighter control of units, E2150 allowed settings of commands like "don't attack this unit unless..." or "attack nearest unit, or weakest unit", and my favorite: "lights on/off/auto"

To a very limited degree, but it would interfere with all the other stuff u want to do.

[*] Oh, and the UCS side of E2150 had the "shadow generator" to shield units from the enemy, I assume this can also be worked into a mod?

Define "shield".

Enjoy.
AmberZ