Ye Olde Inn

This is more about visible monsters or heroes, line of sight is pretty simple.

I have two levels of visibility, direct one and indirect one.

First of all, if your line of sight to a figure, is blocked by a wall, then you cannot see it, period. Cannot shoot at it or target spells at it.

But what I call indirect visibility, is when your line of sight to the desired figure is blocked by another figure(s) only, and not by wall.
Then you can cast spells as normal, but you can't shoot/throw something at it.

Only when you have direct (full) visibility and you line of sight is not blocked by anything can you shoot ("send") something to the figure you want.

In addition, I have divided the main spells into two groups, one group needing direct visibility to cast, and one group needing only indirect visibility.
It just so happens that only "Ball of Flame" needs direct visibility in my games, as it is a fireball that actually needs to travel from your square to the target's.
But with a healing spell, to heal a buddy of yours, you just have to partially see him behind an Orc, to magically transfer your healing energy to him. There is no need for throwing something to him, sprinking him with water or anything of that sort.
For the Fire of Wrath I have decided that you do not need direct visibility, as I feel that it isn't a sort of magic missile, just some magical way of blasting someone with flames, not flames "sent" by where you are standing, but I guess some players might want to think that it is sort of a fireball as well, so clear visibility should be needed.

How does everyone feel about that? I think it enables the Wizard(s) to be more active in critical times of a fight that melee heroes are blocked behind enemy lines.

I ruled this:

Any miniature that stands down line-of-sight is "visible" (by smell, by noise, by anything). If a hero looks down a corridor and the first thing he sees is a chaos warrior blocking LOS to a goblin behind him according to the official rules, I nevertheless put the goblin on the board. It opens up the game in so many ways I won't begin to describe it. Accordingly, ALL objects in a room are in LOS and are put on the board when a hero enters the room or opens the door to take a gander. If I am of the opinion that a monster is hiding behind the door I will implement this. It's only visible when a hero enters.

1) For ranged attacks: If a virtual straight thread can be spanned from the chest of the attacker's figure to the chest of the target figure without any interruption, the shot is allowed. Each miniature or piece of furniture or 3D object that is on a square underneath this thread but does not block the attack (a Goblin, when the Elf shoots at a Chaos Warrior, for example), but rather imposes a malus of -1 combat die on the attack. If those obstacles are enough to bring the attack down to 0 dice, it's not allowed.

2) For spells: A spell can be cast on any miniature (or object) that is visible; that means a spell can be aimed at any miniature in the same room or corridor where the spellcaster is, irrespective of any blocked line between them.

I hope I described it clearly because intention, idea, and implementation of the rule are quite intuitive and self-explaining (the dreaded "realistic" that everyone hates so much in terms of HeroQuest) in our game.

I would say that, if you can see the hero/monster, even looking beyond another hero or monster, then
certain spells can be cast. I would not allow any spells that require the magic to go past the hero or
monster in the way - like fire ball. Fire ball, in my mind, requires a magical ball of fire to go from the
caster's finger to the target, and so real line of sight rules should be used. Sleep, or healing spells, things
like that, would be fine.

The EU rules already allow visiblity to all miniatures within a room. I feel that the assumed movement necessary for this is enough of a boon in most Quests. The relatively confined spaces of corridors and narrow doorways between rooms don't provide this leeway. It could be rationalized that the obcured view of the target lacks the focus necessary to set a spell on a figure's location. Or it could be described as you or knightkrawler see it. Going your way, I'd say include Arrows of Darkness as needing direct visibility.