@Weasel
Regarding artillery, I think a good way to distinguish the need for an FO is to see at what level the artillery unit was attached and how it participated in combat.
Taking German WW2 as example, there is surely no need to buy FOs for mortars attached at platoon level, fire by those units should simply be plotted by the platoon/company CO of the infantry. Same counts for the mortars of the MG/heavy weapon company as these directly support the 1st to 3rd infantry company. Fire by them should simply be plotted by the company CO of the MG/heavy weapon company or the battalion CO which the MG/heavy weapon company serves.
Same counts for infantry guns.
It makes a lot sense to make FOs mandatory for artillery elements that are not part of the infantry or armor assets. Taking Germany in WW2 again, the artillery regiments of a infantry division needs such FOs that already comes from the way they conducted operations.
While a general rule is easy to follow it's in my eyes nicer to differentiate the forces by the way they conducted operations and by the way they were structured, in this case the rules should differentiate between direct infantry support assets attached in platoon/company/battalion level and higher artillery assets.
This helps the player to decide the need for FOs and it also "carves out" the characteristics of a country and it's forces stronger.
@Spledge
I was thinking about the infantry vs tank fire too. Some points must be considered.
1. Is that AFV transporting anything on it?
If so I think it's a legal target because I doubt that infantry would let the opportunity pass to hit enemy infantry that is so exposed.
2. Is that AFV buttoned up already?
If not, I think that a tank that has its hatches open and its commander standing in the hatch is a legal target, also because the infantry can't kill any of the crew unless they are out of the tank, all they can achieve is to button the tank up for the price that they expose themselves.
3. In the rules we have already the rule to Z fire up to 2 hexes away, and it doesn't seem to restrict fire on tanks.
But is that enough?
Overall the question is, at what point does it get gamey to shoot on a tank?
All infantry assets firing like mad over all ranges at a tank just to button him up seems wrong but then a tank driving by infantry positions simply invites to fire on it if unbuttoned especially if it carries infantry assets.
A bit propaganda on that topic:
https://www.youtube.com/watch?v=rLZKbD2Turo
https://www.youtube.com/watch?v=T4eYO_0C5So
https://www.youtube.com/watch?v=M67dgSI-lMw