I have a feature suggestion:
Add a tile property that reduces sight distance, but does not block walking or shooting through. With this a modder could add tiles that represent fog (on part of map), sand storms (on part of map), waterfalls, thin curtains, illusionary walls, etc... for more interesting maps.
I think there are several possibilities to implement this, but I cannot judge how much work it will be:
1. There is LOS mcd (no 31), which can be switched to block line of sight to terrain but it does not block line of sight to units. Change the behavior that it also blocks sight to units (not sure if this would change vanilla gameplay???).
2. Add completely new mcd value which can be added to a tile via ruleset with 'mcdpatch'. The new mcdvalue could be an integer that, if switched to non-zero, reduces sight range similar to smoke.
3. Make it possible that units can shoot through very 'soft' tiles. That means tiles with sight blocking LOFT,but very small damage resistance (zero or less?) do not block projectiles and melee attacks.