A seperate brainstorm forums?
Why not?
But then we'd need to storm on two places at once. I don't think I got the wind.
I like it you like it.

Last time I mentioned supplytrains, they were met with the opposite response. But that was with a totally no-fun logistics concept.
I'd love to have baggage trains in, as they were a major part of early WH versions.
(And campaingns/special defend the train objective scenarios)
I agree with the points you mentioned, still the concept should not be too complex.
Any army has a train. stationary armies just have a HQ in some village. It is frankly the logistics centre, including whores, foodstores, pay, priests, commander, fledgers, mobile smithy, etc.
That said, some units are independent and live of the land or don't need supplies. Such as woodelf waywatchers etc. I would say scouts too, but then I thought about halflings...they and baggagetrains are inseperable.
These units should have a survivalist-skill.
Terrain damage would only be incurred in some types of terrain and each type may have a skill/ability to negate the effect. (Chaos units shouldnt go mad on chaotic wastes, they are already mad.)
...Hmm a nice Chaoswaste effect could be sapping unit morale, then when too low instead of fleeing, they turn Chaos.
A carnisaur transport would replace the train as a special unit and would not have the neg.terrain-modifiers.
Undead still need arrows made and a spare femur to replace the broken one.
But I agree, they should be free from terrain damage or train-healing. However the train could be used to transport vampirecounts and other large artifacts. Nah, better to assume they travel in style while they move about.
Undead armies not always consist of only the dead, so a national ability is no option.
Vampirecounts have living underlings and even Ghouls are a living unit.
So...undead special promotion/ability to give certain properties?
-no healing.
-no damage from disease or naturalterrain or undead terrain.
-cause fear.
-immune to psychology.
-magical instability + -controlled automaton
(seperate ability to distinguish from vampires and liches, can also be used for summoned units)
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Sappers.
Great! Always loved sapping, never much used in standard games though.
The upgradable fortifications (mentioned earlier in terrain-upgrades section) would come in nicely here.
The fist level would be a wooden lookout tower or defensive camp (Roman style)
This camp could provide some small defensive bonus, but more importantly units would not flee away and rather huddle in the camp than run away.
sapers don't build roads. But can build tunnels, fortifications in enemy territory and give a city assault bonus to other units in same stack.
Tower and camp could be upgraded later.
wooden lookout, -> stone-tower, -> small castle or wizard tower.
Entrenched camp, -> wooden pallisades, -> small castle, -> Fortress.
I would assume potholes, wooden pikes, sandbags are covered by the standard fortify bonus of fortified units.
The towers increase sightrange of occupying units, but can only offer defensive bonus to a small number of troops. Maybe towers could be assumed to be always manned and give LoS.
So either a small bonus only for all or only for the first unit in the stack. (could units be 'loaded'into terrain improvements?)
The camp-series of fortifications should prevent troops fleeing and give some addional artillery abilities. As well as negating any deadly terrain effects in that tile.
However camps do tend to be a magnet for filth and diseases. So a negative health if within a cities footprint. Small chance of units contracting an ailment, that should be temporary. Once a unit is diseased, next turn either healed or a tiny chance of a plague taking hold. A plagued unit remains sick and contageous untill healed by a unit able to heal.
(The diseased ability in FFH was too pervasive, one battle with a diseased unit and virtually all unit in both piles were sick.)