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Here's some info about how common character conditions are processed in the game:

Blinded: A blinded creature's armor class decreases by 2. It also loses any dexterity bonus to armor class. Speed is decreased by 50% and the creature loses the ability to charge. Whenever the blind creature attacks an opponent, the opponent is considered to have concealment 50%. Lastly, a blinded creature cannot take AOOs (attacks of opportunity) against other creatures.
Graphical representation: a black eye icon floating over the creature.

Sleeping: A sleeping creature has been magically put into a deep state of sleep and is unable to act. The dexterity score falls to 0. The creature is considered helpless and is therefore a potential target for a coup de grace. Helpless characters victim of a melee attack suffer a penalty of -4 to their armor class. The spells Sleep, Deep slumber and Power word sleep can give this condition to a creature. If there is a friendly creature near the sleeping creature, the friend can shake the sleeping creature out of its sleep with the Wake Somebody Up standard action. A sleeping creature that takes damage is automatically woken up.
Graphical representation: “Zzz” symbols floating over the creature.

Stunned: When you are stunned, you cannot take any combat action, your armor class decreases by 2 and you lose any dexterity bonus to armor class. Spells like Sound burst, Discharge and Power word stun can give this condition to a creature.
Graphical representation: stars rotating over the creature.

Confused: Each round, a confused subject rolls to determine what it will do: Attack an enemy creature (01-10), Act normally (11-20), Do nothing (21-50), Run away (51-70), or Attack the nearest creature (71-100). The spells Confusion and Insanity can give this condition to a creature. The spell Heal can remove the condition.
Graphical representation: coloured rings floating over the creature.

Protected versus evil: This condition gives a character a bonus to armor class and to saving throws whenever he his attacked by a creature of evil alignment: lawful evil, neutral evil or chaotic evil. The character also gains immunity to mental control. The spells that give mental control over a creature are: Control Person, Control Monster, Dominate Person, Dominate Undead and Dominate Monster. You gain the Protected from evil condition with the spells Protection from evil, Mass protection from evil and Greater protection from evil. Evil creatures instead cast the spell Protection From Good and
gain bonuses when attacked by creatures of good alignment.
Graphical representation: a blue aura surrounding the creature.

Paralyzed: A paralyzed creature is frozen in place and is unable to act. The dexterity score falls to 0. The creature is considered helpless. Therefore, it is a potential target for a coup de grace. Helpless characters victim of a melee attack suffer a penalty of -4 to their armor class. A paralyzed spellcaster can still cast spells with no somatic component (use the Metamagic Still feat). A paralyzed monster can still use its Breath weapon. Some creatures, like the Ghoul, have a special attack that paralyzes the victim. The spells Ghoul touch, Hold person, Mass hold person, Hold Monster and Mass hold monster can also paralyze a creature. When the paralysis originates from a “Hold” spell, the victim can attempt to Break Free every round as a full-round action. This requires a successful Willpower saving throw.
Graphical representation: blue rings around the creature’s waist line.

Nauseated: Creatures with this condition are unable to cast spells or attack. Nauseated characters can still move. The maximum distance moved in a round is equal to the character's speed. The spell Heal can remove the condition.
Graphical representation: the creature turns green.

Invisible: When you are invisible, you gain a +2 bonus on Attack Rolls when fighting against sighted opponents. When you attack them, sighted opponents also lose their dexterity bonus to armor class if they had one. If the creature you attack is blind, has the True Seeing condition, Can See Invisible ability, Blindsight ability or the Blindfight feat, you do not gain these bonuses. Lastly, when invisible you gain concealment 50% and enemies cannot take AOOs against you. If you received the invisible condition from the level-2 Invisibility spell, you lose the condition as soon as you attack someone or cast an offensive spell.
Graphical representation: the creature becomes transparent.

Enlarged: A character can gain this condition from the spell Enlarge person. The condition brings the following effects: strength increases by 2 (size bonus), dexterity decreases by 2 (size penalty), the armor class decreases by 1 (size penalty), Attack Rolls decrease by 1 (size penalty). Damage Rolls increase greatly as the weapon's size category increases by one.
Graphical representation: the creature’s sprite is stretched by a factor of two.

Comments are welcome. For example, should the game allow to perform a ranged attack while blinded?

Hmm...on the blinded question at least:

Yes. Yes indeed. However, there should be no control as to WHICH target winds up getting attacked, if not perhaps the afflicted character itself. Like fighting in the dark essentially.

Unless you just take it as a sided measure, like one eye being damaged or something.

Okay. Assuming a blinded character tries to shoot at an orc, and there are no other creatures present, how should the game deal with this?

Currently there's just the 50% miss chance from total concealment.

Assuming there are several creatures in the targeted direction (for example the blinded character is shooting at an orc who's engaged in melee with an allied fighter), do you think the orc and fighter should be selected at random to determine who's hit?

Hmm...do missed missile implements remain on the floor? If so, it could be beyond a simple "miss" of an enemy under normal circumstance and instead the arrow or whatever could plink against a wall perhaps a couple spaces removed from the target. Or there could also be a chance of simply failing to properly fire the bow, "missing the quiver", "failing to string" etc due to not being able to handle anything with finesse whilst afflicted.

In the second situation, I reckon that, providing what I list above is kinda in play, we could just say all targets are eligible ones...perhaps with a weighted chance on hitting an ally accidentally if under the influence of multiple forms of blindness, bad resistances to it, and so on while better odds of hitting at least some manner of monster if not just a simple "miss" otherwise.

Right now if you miss you lose your arrow. Well I could have a rule saying that if your character fails the concealment check due to being blinded, and there are other creatures in the general direction you're shooting at, then there's a 50% chance that you hit one of those other creatures rather than a plain miss.

Another thing that might be fun would be that if you're trying to cast Fireball, or any damaging spell, and your spell is interrupted because you took damage then the spell explodes with you at the centre. However according to the rules you would just lose the spell.

Hmm...in terms of the Fireball example something else that might work to similar lines would be an "power level/overload" aspect in terms of spellpower similar to the one featured in Wizardry 8. Unlike Wizardry 8 though, where direct damage spells became all but worthless by endgame it seems, I could see such a notion adding another rich layer of strategy and risk/reward to spellcasting in general in this game.

I like this general facing direction blind shot notion, seems more realistic to cite as opposed to accidently turning around to shoot a distant companion while blinded looking straight ahead at a crowd.

Yes. In Wizardry 8 the paralyzing and confusion spells were often more useful. In terms of power level there is already a mechanic in the rules actually. You can apply a metamagic feat - empower, heighten, widen or maximize. Empower increases damage by 50%, maximize sets the damage dice at the maximum, widen doubles the radius of a circle spell and heighten increases the difficulty class. Also you can take feats like Spell Focus to increase the chance of a spell actually working.

I was also thinking about the shooting mechanics. There really is little realism in D&D's rules, for example, a character has the same probability of hitting a halfling 10 feet away or another at the limit of the bow's range. You must beat the armor class and it doesn't depend on range.
Also imagine you're shooting at a halfling wizard 50 feet away and there are two ogres standing on each side of the halfling. In reality you would have a very high chance of hitting one of the ogres, but with the mechanics as they are it's either you hit the halfling or it's a plain miss.

Even in Fallout as far as I know the game did not take account of the margin of error - your bullet was always assumed to take the right trajectory, while the targeted creature could dodge. Anyway it's not the current priority to upgrade the combat engine beyond the OGL rules but I might look into it later on (particularly the blinded case). Thanks for the discussion.

Blast...and here it was only a few months ago that I read all of the manual to ToEE and keep forgetting the inspiration drawn from it with this.

Glad to have the bounceback of ideas and notions. The strength of this project, and I would hope future ones, is that you are not set on dogmatic adherence to an existing ruleset and allow for variance and unique flavor---a style that bodes very well for creating rich content in the future.