For example for magic, many have said "there should deffinetly be a chance of failure for magic" but how exactly, do you mean a failure by statistic chance like in Morrowind or a failure by certain factors and how exactly should that failure look like (complete fail to cast, only partial success, only fail under certain conditions...).
So basically instead of just saying "it should have fail/no fail" try suggesting a actual system on how the skill could work.
Magic is a good example why certain aspects of the game need a chance of failure, in my opinion. The reason is that it is very easy to cast a spell for the player - he just presses a button. However, for the character, it probably isn't that easy. Generally, when we think of casting spells, we either think of mumbling magic words, or waving your hand/staff/wand in a certain way, or both. If that goes wrong, then there are certain possibilities:
Effect 1: The spell simply doesn't work.
Effect 2: You actually cast a
different spell (possibly one you didn't even know).
Effect 3: The spell backfires somehow (you do manage to summon the creature, but it is hostile towards you; you do manage to cast the fireball, but it explodes in your hand; etc. pp.)
Now, this is a failure that's on your character's part, but it's nearly impossible to create a system that makes YOU the one who screwed up. Arx Fatalis made something like that when it required you to perform the movements of the hand yourself, but at least I found that very annoying - and it only covered failure effect 1 from the three I just mentioned. With a system like that to cover effect 2, you'd need to put in thousands of spells that have very equal hand movements - increasing the annoyance of the system. I can't think of a way to implement effect 3 other than chance.
So all in all, there doesn't seem to be a good way to simulate casting a spell. Oblivion simply said "Okay the spell always succeeds, but it's weaker than it could be if you have a low skill". That's certainly a possibility, but it takes away all the dangerous fun of sorcery. Magic needs to be "unpredictable" in some sort of way, because this is how we know it - especially the very skilled wizards usually tend to create spells that are beyond their capabilities in popular fiction.
Okay, so that's why I'm all for a dice roll system for magic. And this is how it'd work:
First of all, the chance of success would calculate itself exactly like it did in Morrowind - based on Willpower, Fatigue and skill in the school of the spell in question. Now depending on success or failure, different things happen:
Success:You are able to cast the spell. Now that you've managed the first hurdle, it will be easier for you to hold the spell, because you know now "how it feels". Now, a difference to MW as well as OB is that the duration of the spell is not fixed. You can set it to something before you cast it, or after casting it, you can use some buttons (your mousewheel, whatever) to shorten or to lengthen the time. Each second you hold the spell, a certain base magicka cost is taken from you. This will allow for a much more flexible use of spells, and makes more sense than constant recasting of the spell. The spell will naturally end as soon as you have no magicka left. However, after a certain "care-free" amount of time (dependent on how easy you managed to cast the spell), there will be dice-rolls if you can hold the spell any longer, which will at first be more lax than the one you had to win at the beginning of the spell, but with increasing time, they will become more harsh. After some time, even a wizard with unlimited magicka wouldn't be able to keep the spell going. Anyway, these dice-rolls will lead to a belated failure.
Failure:For the failure, it doesn't matter if you failed right at the beginning of the spell, or later (only for you it will naturally be better if you were able to hold the spell for some time). All that counts is that you failed, and
how severly you did so. For example: Your chance of success was 30%, and the dice-roll came to be 29. This means that you only barely didn't make it, which will increase your chance of getting a failure that is not so bad (effect 1 from the three options I mentioned earlier). If you, on the other hand, only rolled a 1, then chances are good you'll get a backlash (effect 3).
As far as I'm concerned, this can be a very easy calculation, because it only decides which punishment you get.
After that's decided, the severity of the punishment will be calculated. Since effect 1 means simply that the spell doesn't work (anymore), there is no severity, and you're lucky. Effect 2's negative side is that the spell you're casting svcks magicka out of you just like any other spell (and you can't shorten the time for this one!), but on the other hand it might be a useful one. The really severe punishments result from effect 3 - backlash. There's a mod called "Sorcery's Toll" for Oblivion, that's basically what I'm thinking about here. Magical diseases, loss of fatigue/health/magicka, damage of attributes, damage of skills, summons that turn against you... the possibilities are endless. Effect 3 is basically the "critical failure" of my system, only that because it might turn out to be not super-bad (if you're lucky), it's a little more probable than the usual critical failure. And of course, chances for really bad backlashes are higher for more difficult spells (i.e. where you try to cast a spell that's way beyond your skill).
That's my idea for a fun magic system.