Throwing out some ideas here. Skirmishers cannot move after attacking. Higher RCS and CS compared to now. Their niche would then be something like mobile firepower. Contrasting this with mounted, which can move after attacking, but take damage during the attack. We have done something similar to naval ranged, and it seems to have worked well. If the problem is that skirmishers can fire from rough terrain, then retreat behind it, such as shown by Pineappledan's image, what if they can't attack when in rough terrain? This is a much more extreme version of -RCS in rough (or +RCS in flat), but it prevents the malus (or bonus) from being swamped by promotions. Both would deal with the issue of skirmishers being too good in rough terrain.