ToME WikiRavenRed: Just a quick (and possibly ridiculous) idea.
Right now, all spells and magic devices are "tracking", which is to say that they will always hit their intended target. What if they actually needed to be aimed by their caster?
My idea is that there should be a new skill of Spell Striking (under the Archery tree) which determines your success in hitting with a magic bolt or ball (note that fireball effects could still kill from the blast in any case).
The chance to miss should perhaps be something like (with apologies to Zizzo's magnificent Killerbunnies spoiler)
AC/5 in Skill-Distance+Luck
This is a gentler progression than Archery, although it DOES discount to-hit bonuses.
A miss should throw off the "end point" by 6 - Skill@5 squares for beam + bolt spells, and should throw off ball spells by 10- Skill @ 5 squares in TOTAL (on both X and Y axes). As previosuly mentioned, splash damage will often have a devestating effect in any case.
The skill should be available to Mages-types at a mod of 2.000 wheras others should have it at 0.800 or so (starting scores of 2.500 and 1.000). Since this would also be applicable for wands, it might pay for most characters to put a few points in it.
In order to compensate for this for players, all targettable spells should have their mana costs reduced by 20% or so. This plays into the credo of mages being hit-and-run artists, and whilst it gives additional complexity, it also makes for a slight check on the power of mages as well as something for other classes to think about as well...
2c worth and counting...
ZasVid: Man, you're mean 
I always thought that fail rates are a substitute for missing spells. This seems reasonable, but I don't like it. It would really nerf spells like Confuse if it applied to them -> to get the effect, you would have to not fail the spell, not miss it and have the monster fail its saving throw.
RavenRed: Moi, mean? Have you been talking to Neil? 
This could be made a flag on damage-causing spells rather than effect spells, but as I said, the 20% mana reduction would mean more bang for your buck in any case. More bang, just slightly less accuracy... (and more possily fun misfire effects)
NeilStevens: Even if the miss rate were only 20%, this would still make the game harder, because it would take longer to do the same amount of damage to a given target. And I didn't comment at first specifically because my major reaction is "That's mean," and I said that enough lately, heh.
Raven"Mean"Red: True, with the (sizable) exception of Noxious Cloud. Howabout the Miss Chance being AC/8? At Skill Level 50 (which would cost a mage type 48 skillpoints, excluding Lost Sword quests) there is an approximate 41.67% miss chance at AC 150 (Morgoth) and distance 5. The highest AC I could find was for the void spirit (of dragons / or CON, from memory) which was 190. This still leaves a 48% chance to hit, which is not absolutely unreasonable for the toughest (by AC) monster in the game. Especially considering that the mage would have 0% fail rate and truly heinous damage by that stage. The other thing to consider would be implementing a "miss" effect for monster spells, which would make life exceedingly easier for high-AC characters. What's bad for the goose...
(Oh, and I never object to you telling me I'm mean, as long as you're constructively telling me I'm mean
)
TheFalcon: I'm not sure the idea is "mean" necessarily, but it does have the effect of forcing an even wider spread of skill points. I'd prefer it if magic skill itself included some kind of accuracy test, since it seems rather excessive to add another skill to invest in. On a related note, how does splash damage work at the moment? Is it graduated with distance from the centre, or is it uniform?
RavenRed: Actually, if you substitute "Magic" for "Spell Striking" that would make this idea a hell of a lot more workable. Considering that Magic only (at the present moment) affects Max Mana and Psuedo ID, that would give yuo a better reason to pump Magic in the midgame, rather than just the spell schools with an occasional skill point bone thrown to Magic...
Actius: I thought this is what the saving throw was about?
Kn: I think this skill should be called aiming or marksmanship and should affect both archery and spells. And it should only affect direct attack spells, not curse spells like confuse. It would mess up balance, but hey, the game is already not balanced.