Elves could get a magical time freeze, stopping everyone's unit movement and production for a day or so.

It would cost 0 mana because everyone knows we wouldn't abuse of this capacity.

 

While I'm not outright opposed to airships in Illyriad, I'd prefer the flavor of air travel to be more mythical and less technological (i.e. dragons, gryphons, harpies, pegasi).  They would naturally limit capacity.  The need to have a near one to one mapping with ground units (i.e. make an infantry unit into a dragonrider by adding a dragon) would balance the cost with the benefit.

 

As for ground based transport, I think it would fit in with the world to just be able to mount the whole army on horses.  It would work something like this:

 

When on the "Send Army" screen, there would be a check box allowing you to mount your army, which would impact the speed with which it would move.  It would require Horses = Army Size - Mounted Units.  Since the foot units are not trained to fight on horseback, the extra horses wouldn't impact the outcome of the battle.  If your army returned/survived, the horses would stay with the army until entering your town at which point they would return to your supply.  If your army were eliminated, the horses would remain with the victor.

 

So you would have the option to make moving your army fast, but it would be costly and potentially end up gifting a lot of horses to your enemy.

land: 

   same units as now

   ground transport:

      humans:

         wagons or carts.

      dwarves: 

         land sailers (basically a cart with a sail on it)

      elfs:

         land based dragons

      orcs:

         trolls pulling sledges

         

air:

      elfs: 

         giant birds

      humans:

         gliders

      dwarves:

         hot air balloons

      orcs:

         flung from a catapult (I cant think of anything better at the moment)

underground:

      I really don't think that there should be an underground, for these reasons:

         1: it will be hard to figure out where to put the best tunnel entrances to benefit all players.

         2: large armies could simply cut off all entrances for newer players, resulting in less fun and more inactivity.

         3: who wants to have armies getting lost in the dark without a light?

in conclusion, land units should still be the dominant units, but there should be 1 type of air unit to add another dimension to the game.  also aircraft carriers would be AWESOME!!

Multi level of game does seam nice. 

Here is some of my ideas (I tend to give ideas like that so other people can build up from them using their own Idea) :

-Darves should get a bonus to tunnel digging, but every species should be able to.

-air unit should be very weak, just moving on top of a tile where enemies ranged units are should be enougth to sink them (thats actually true most of the time)

-boat travel time should be inacurate, as a storm could delay and a good weather could speed it up.

-Some tunnel would be there from the beginning (make it the ruins of an ancient civilisation) but other tunnel could be added.  

-How about making the tunnel upkeep in this way; for every tunnel type (depending of the type of tile it crosses) there would be a 1/x chance that the tunel collapses every day.

collapsed tunneltile would have to be redigged. Long tunnel would obviously end up having a lot of tile to be redigged every days.

Collapsed tile wouldn't stop stuff passing through, but it would slow down movement so much that you would always want to keep your tunnel as clean as possible. (this way you dont complitely stop your caravan if the tunnel collapes on front of them after you loged off.)

Tunels under mountains would have less chances of collapsing but would be more expansive to redigg. tunnels under rivers would be very short lived and would need a lot of maintenance.

it would take some time to tunnel each square, say a 2 minutes+or- a couple seconds for different types of tunneler (this seems not that much, but if you tunnel out 100 squares that is 200 minutes, or 3 1/3 hrs).  once a tunnel has been dug, regular armies could pass.  

there should also be different types of soil and rock:

Those suggestions are assuming transports can hold tons of units. If each transport was basically putting the soldier on a horse, I could see just adding about 5 speed and 1 upkeep cost. Maybe 10 horses for 3 extra speed and 10 extra upkeep for siege engines.

You could also call this transport some sort of herb that speeds up units, so that if could affect cavalry too.

 

Just putting an army on horses is also a very bad idea. Putting untrained units on horses is sure to get some of them killed, especially the heavier armored ones. Not to mention the army would be sore after riding for a questionable amount of time, reducing combat effectiveness. Add in a moral penalty and you have a useless army (some of them DIED on those horses). Horse units would also be impeded by forest and mountain terrain, so they still wouldnt get there faster if they had to go around a mountain range instead of marching through a forest (also add in more deaths the longer the time traveled). Actually trained calvary units would still move as normal and not die, because they spent time training for riding horses (or wolves).

+1
 

They wouldn't be figthing on the horse, they would only travel there on them. They don't have to be fully harmored when travelling so I dont understand why you so desparatly want them to die on those horse (unless if you dont have enougth horse and therefor dont want others to gain this ability)

I forgot to mention in last post that maybe we could have cow to carry the twice the equipement that an horse can carry (since cow are much more valuable in game)

the spell could be cast from the military summary, in much the way you check "feint".  It would cost about what a regular spell costs (10 mana p/h, 3 books 2 cows) and last until your army reached their destination.  

more horsepower = faster engine right? eh?

A) hurt

B) dead

or C) not fit for combat

-I don't know what you think you know about horses, but There isn't a horse alive that can gallop very long with a guy on it's back. Galloping are only used by cavalry to charge the enemy (it's a scare tactique.). People traveling on horse back let their horses walk for pretty much the whole way (except in movies). A walking horse will have time to avoid you if you fall and you won't get trampled.

- I guess that you fell off a horse when you were young, and it obviously left some indelebile mark on your subconscious.

you dont want us to be able to put our army on horseback. Please explain why instead of just insisting on giving us weird and unjustified objection.

don't both A) and B) fall under C)?  also, there is still the problem of what to do with the horses when you reach the battlefield.  do they get slaughtered as meat? do they get slaughtered by the enemy?  do they stampede randomly, creating chaos and havoc wherever they go?  "cry havoc, and let slip the horses of war!"  

Horses dont influence the outcome of battle as non cav units dismount before engaging the enemy.

all that I have to say is, keep up the good work!

Or they can march at normal foot soldier speed so they dont run anyone over, rather than a light trot for getting somewhere (forced march anyone?)

Just a thought