I know we have gone round and round on skirmishers before. For me the issue is niche protection. It seems that each change pushes the skirmisher against another units territory, so that the skirmisher is either superior to the other unit (and therefore, why build the other unit) or inferior to the other unit (and therefore, why build the skirmisher?).
In the earlier version, I felt skirmishers stepped on both the melee horse and the ranged units roles. The skirmisher was as mobile as the horse unit, and while it didn't do as much damage it did not have to push into dangerous spots either. Further even though it shared the city attack penalty, because it took no damage from cities it was still effective at softening them up. On the ranged side, it did more damage than an equivalent ranged unit, could hit places a ranged unit couldn't (aka into rough terrain spots), and could retreat far better than a ranged unit could. So it was the dominant unit to build. I personally have no desire to return to the original skirmisher, it was simply too good.
So then we added the rough terrain penalty. Now I feel the pendulum was swung the other way. Ranged units do more damage (and are cheaper), and hit about the same place a skirmisher can hit. Melee horse gives me more mobility, damage, and tank ability. So I don't see a strong need to build skirmishers.
In my head I can see a few ways to go with skirmishers:
(Note: Some of my ideas steal from certain civs. I know some people's immediate reaction is...that will totally buff/nerf Civ X". Frankly, I always value standard balance over a single civs. I would much rather come up with a good idea that works in general, and then adjust a single civ to compensate...than give up a good idea because one civ already uses it)
1) Return to their original mobility with lower damage. So I buy bows for the power, skirmishers for the mobility. And the difference needs to be pretty strong, I'd say at least -25% CS compared to the bow equivalent.
This is probably my favorite, and it's one I favored early on. It's simple, requires no changes to overall strategy and likely effective. I don't know if 25% is enough of a CS nerf, at least on both Skirmisher variants.
2) Original mobility and power....with a significant cost increase. Effectively these are your cream of the crop troops. They are your most powerful, but with a significant investment vs just getting bows. If you wanted to go more costly here, you could make them cost 2 strategic resources.
This is also good, and would effectively hamstring the most ornery problems of the Skirmisher line. If we did go this route, I feel like unique Skirmishers probably all need to stay on 1 Horse; it's already disheartening enough to have your UU locked out by strategics as it is.
3) Give them 3 speed but ignore terrain penalties. So less mobile than horseman but consistent mobility in all fields. To me we are likely still back to the old "untouchable" problem here, but its an idea.
Nah. This means I still move into a hilly forest, shoot and move back out, which IMO was one one of the biggest issues with them.
4) Drop their attack entirely, giving more CS, and give them the Pilum ability (10 damage to all adjacent enemies). This is a more extreme idea to give skirmishers a different niche. Effectively they are engaging with the enemy (aka skirmishing), and gain an area attack of a sorts (representing their high mobility to hit multiple places). However, unlike the original model they have to stand their ground, and so counterattacking is possible.
This is radical; I love the flavor, but it sounds like hell to balance. This seems like all kinds of problems, but a part me of really wants to test it.
5) Low damage splash attack (aka the Arabia camel archer as a standard). Similar to number 4, the idea of area damage to represent their high mobility and ability to hit several places at once. This would be more traditional than the pilum idea. So skirmishers would help soften up armies, but you would still want bows to do the "real" ranged damage.
This is an interesting one. Not sure if I have strong feelings on it.
6) Give them something that grants them double bonus from flanking or something. Aka skirmishers are normally pretty weak (in general not worth using compared to bows), but if the enemy is tied up (aka you have a lot of flanking), than skirmishers do a tremendous amount of damage, as the enemy cannot avoid them. So this gives me a specific niche on the battlefield that has to be earned.
Eeeeeeeh. I can see this working, but I don't think it interests me personally.
7) Skirmishers get ignore zone of control by default. So even if rough terrain ties them up a lot, they can still get in and out of certain places more easily than other units.
This sounds like a nightmare. Playing with quick combat, you'd be having units at the back sniped off and have a very hard time figuring out where it came from.