The right rule for starvation is tricky. I want just one rule for both living in town & the adventuring. Using this same rule, city populatoin decreases quickly after it runs out of food. You can imagine the area around the city run out of food for citizen to forage, or they just return to rural area to forage, or some just dies of starvation.
I like a simple zone of control (ZOC) system. If there is ZOC this starvation mechanism should make siege work too. Siege should then cut off citizen's foraging around the city, causing starvation happens quicker.
If there is no Empirewise morale mechanism (& a way to decommission troop to civilian), I'll say AI always decide to starve Civilian when food is limited as troops are more expensive to produce/valuable.
I've updated OP to reflect our current thoughts. The old OP is kept below.
Most units should have a limited range of activity on the map. They cannot wander too far away from their natural habitat, without some kind of supply. Most units should not be allowed to walk across an XXXL map, or fly across ocean unassisted.
Basic ability like “walking” should be implemented like the following.Walk X turns
The counter X is reduced by 1 each turn. It is recharged completely when the unit walk thru a friendly settlement (e.g. City/Village/Castle/Den/Lair/etc). When X reaches 0, the unit will be "Exhausted". Exhausted units will have Att, Def greatly reduced & movement speed halved.
These units can still keep pressing forward, but the risk of defeat is huge. Generally speaking, exhausted units should retreat, but it is a choice up to the AI/Gamer. The fun thing about Exhaustion is, since it is a status, it can be removed by spell/item/heroes' skill when appropriate.
If the friendly settlement is a recently captured one, there is only 20% chance each turn of recharging X. This is to slow the progress of Stack of Doom.
Natural Habitat: Dessert/Forest/Grassland/Sea/Land/Sky/Underground/etc
When a unit ends the turn on one of their native Natural Habitats, the counter X is recharged to its maximum value.
Seafaring X turns: Similar to Walk X turns, that resupply happens only at a friendly settlement.
Fly X turns, Swim X turns
Similar to Walk X turn. Most, if not all flyers or swimmers' natural habitat is a land tile. The unit drowns when X=0 and it cannot swim; or suffer some kind of damage when they crash land.
A way to simulate supply line
At the end of each turn, all units' counter X equals to any unit that has a higher X, as long as it does not exceed its maximum possible value.
For example, assume unit B has X=2 now, while the maximum possible is 5. A new reinforcement Unit A with X=8 joins the stack. On the next turn, Unit A has X=7, unit B has X=5. Unit A acts as ‘supply’ in this case, allowing all other units in the stack move further.
This means at most of the time, gamer need not worry about supplies. They need to, only when the battlefield is so far away from their road/border/natural habitat /settlement. They'll need to find that Unit A creatively if they want to prolong the war. For opponent, trying to destroy unit A will be very profitable.
Ideally, if there is implementation of Racial Settlement, and the Diplomacy mechanism, this will work very well. Each player has a limited range of activity close to their homeland. They need to capture/establish a new settlement to extend range beyond their natural ones. Scotch earth strategy will be well effective then. Island like Hawaii will act as a very important strategy location, as it will be a place of naval re-supply.
The concept itself is very simple (except AI need to understand it), but it'll offer more strategic depth.