I maintain that lowering RCS, raising CS, removing the move penalty and keeping moves at 5 is the most effective way to make skirmishers handle differently from archers without having to train the AI on new mechanics
Logistics sounds like the most reasonable alternative of all the ones we've heard, but I'm not convinced there's a way to balance the damage and XP potential without making the unit's power output pitiful, or lowering their XP gain to 1XP per action. Any new pillage mechanic isn't worth the energy to teach to the AI, that's why the XP for pillage promotion was taken off the scouts.
Much has been made of the ability of skirmishers to move into rough, hit, and retreat. I find this entire line of argumentation a bit daft because ranged units can already do this. Archers can hit things 2 tiles away, even through rough if they are positioned on a hill. There is no difference between this:
and this:
None. The only material difference in that terrain is damage output.
Skirmishers can do this in flat rough too, but Enough ink has been spilled on these peripheral considerations
With 5 moves, however, a skirmisher can move 2, hit, and move back 2 in open terrain. No need for a movement penalty any more, because this:
Is better than this:
Archers can still hit 3 tiles away, but they can't retreat back 2 tiles. That's where skirmishers excel. That's their comparative advantage, and they don't need a movement penalty in rough terrain for that to exist. Archers already can shoot through rough terrain and they get a 25% defense bonus in that terrain too. There is absolutely no need to make the rough/open dichotomy between these units more obvious than that.
The other major difference, outside these tactical attack moves, is the raw movement difference between 5 moves and 2. Skirmishers can give much larger scopes of vision, and can reach fronts much faster, and can pillage more than 1 tile. If you lower RCS on the whole line then you are waiting for the real damage potential of siege and archer units for the real fight to begin.
The only thing a movement penalty does with 5 moves skirmishers is limit their traversal potential from this:
to this:
I don't think that's necessary, and I think it hurts skirmishers' flavor by making them road warriors, while most of the cultures that deployed mounted archery units aren't exactly known for their infrastructure projects. However, if people really feel that 3 moves in rough is really that big of a deal then fine.