The same problem is that players have no province left, but they troops can fight after days without supply like nothing happens.

The moral of a troops without supply must be reduced every day-switch by 2-5%.

I've found that a player without territories cannot give orders to their units.
A player without resources seem to be unable to repair any damage to their units.
The further you are from the capital, the more the morale penalty for the provinces
A defending player can easily cause huge casualties to an attacking player just through placement and using the correct number of units. For example, a heavy armour column of many tanks and aircraft CAN lose to a bunch of infantry or commandos, AT, Artillery and AA that have dug into a city. Delaying any further advance by maybe days allowing the defender to move troops up to take advantage of the now pinned units.
It's not impossible, but it can be hard to pull off.

kalantigos, that was barely on topic. he wants a system that makes a nation that is too drawn out suffer morale. You are simply giving him strategic advice that can apply to many situations, not just the one described, and has almost nothing to do with what hes asking. and unless the game mechanics have changed in the last few months, non province players can issue orders. Playing as france once, i was destroyed by poland and spain. I had several naval units near polish occupied hamburg, as well as a few land units. My units were losing morale, i beleive, but i was able to capture hamburg for a few hours with my remaining land untis, and switch the targets my naval untis were firing at. poland eventually took back hamburg, though

Nope, they can give orders.

On toppic:
Implementing supply lines to slow down progression. I don't know...

Actually defeating a player that puts all his units in 1 stack isn't that hard. Pick your moment to fight and invade with little groups of units. He might split up in order to save something or will continue and lose everything for sure.

Have you actually seen this in game though? by that time, I dont think any player could possibly have units left, and any they did would be on very low morale. so, i guess supply lines-or something to the same effect-are in game, cause units dont recover HP as well in lower morale provinces, and the farther from the capitol, the less morale provinces have. but yes, the strategy and tips posts were useless. he is asking for something to be implemented, not advice on strategy

If we would need supply lines I think we would even see more huge armies marching forward. Players that march forward with many small armies will need to build a supply line for every army they march forward. Or all would run out of resources. I don't know if it would be possible to implement supply lines for big armies only and not for the small.

If we use some imagination, you can imagine the conqueror plundering the supplies found in the enemies provinces that it conquered (fuel, food, new shoes)

think germans in barbarossa. one of the reasons winter stopped them in their tracks was because they no longer were able to plunder the wheat fields.

this also lets you use different tactics like cutting troops off from the capital. however there are many problem that come with that some thing doesn't go wrong