Here's a little something I wrote up before civ4 even came out that I'd still like to see implemented. I was planning on doing it myself, but I don't have the time for that anymore...
Am I the only one that thinks exploration & expansion (E&E) is too fast in standard civ games? I switched to Rhye's mod in order to experience slower E&E, and expansion is slowed significantly there, but I still feel exploration is a bit too fast. It's still far too easy to circle your continent with ships and map the interior before the middle ages. A supply system would fix this, by limiting the operational range of units. I've seen many proposals, most of which are good, but too complex or too micromanagement (MM) intensive for most players. I've never seen a proposal similar to this one however.
The Basics:
1. All units have a new stat "Operational Range" (OR)
2. Moving more than a number of tiles equal to the units operation range outside of your borders causes a chance that the unit will automatically disband - think of it as "sinking in sea" for land units. For example, if a unit's OR is 4, moving 5 or more tiles outside of the civ's borders would cause a chance of loosing the unit.
3. Technologies and Promotions could grant bonuses to OR.
Using this system is advantageous for several reasons:
1. It limits early E&E without artificially restricting movement (suicide runs are possible).
2. It reuses a familiar concept (ships sinking at sea).
3. No MM at all.
4. This might reduce the desire to use ICS, since you will want to push your borders as far as possible in the early game to increase OR.
5. It adds just the right amount of realism and playability. (IMHO, of course
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Additional Considerations (Optional)
1. Lack of Supply (cut lines, foraging and such)
2. Extended OR (Supply Wagons)
#1 requires #2, so I'll do it first. Supply Wagons (or whatever name is given to them), would be a physical unit on the map, which has it's own OR from the civ's border, but extends OR for other, non-Supply Wagon Units, so that one cannot simply make a chain of Supply Wagons to cross the continent. This requires a bit of extra work to implement. It's completely optional, and the simple supply system would function just fine without it. With that said though, it would be a very nice option to have.
Cutting supply would be a simple matter of killing the Supply Wagon. Then we have three options:
A. All units that were in supply by the wagon are now out of supply and might very well disband next turn. It's not automatic though, chances of auto-disbanding are the same as for normally leaving supply.
B. All units that were in supply by the wagon are given a number of turns equal to OR to get back in-supply (whether by wagon or getting closer to home), otherwise the civ risks loosing the unit as normal. This represents foraging to a limited extent, and could be complicated by terrain and other factors, but I'll leave that to your imaginations.
C. Teleport all units, on the following turn, towards the mother civ, just enough to re-enter supply.
Option A requires no amount of additional work. Options B and C require some extra work, but option C reuses the teleporting concept when you are forced out of another civ's borders. Personally, I would go with option A or B, A before B even, since it requires no extra work.