There's a lot that can go wrong with magic systems for different games. Balancing flexibility, flavor and power can lead to all sorts of problems which can leave spellcasters either stomping other characters flat or sitting in the back of the room waiting until they can do something remotely helpful. Probably one of the hardest factors to balance is divination magic.
It's a pretty essential component, after all I don't think there's a single historical or mythical form of magic that doesn't have some form of prophecy or fortune-telling. The ability to see what others cannot practically defines some forms of magic, after all that's exactly what the omni-present suffix "-mancy" actually means, it refers to a method of divination. It's just morphed into a way to say "Magic of X type". I blame necromancy, it started as just a nice way to talk to dead people and it became responsible for a whole generation of linguistic drift. Tsk. Tsk.
Anyway, so what was I saying? Divination is pretty much essential for any magic system but it's probably the hardest to actually define and balance. It's easy to work out the potential power of a fireball or lightning bolt but there's practically no way to calculate the potential of being able to peek even a short time into the future. The biggest problem is of course that in order to for the power to actually work in the first place the GM has to have some idea what the future holds and I think we all know that when dealing with a group of heavily armed homeless psychotics (i.e. PCs) that's practically impossible.
Some systems try and compensate for this by making divination magic unreliable or exceedingly vague and cryptic. It's even worse when the effects of fortune-telling is left entirely in the hands of the GM as a means to provide "info-dumps" or quest hooks for players. That would be fine but usually the player in question still has to pay some points/skills/etc for what is essentially a shortcut for the GM. These solutions end up either being entirely insufficient (allow diviners to play merry hell with game balance and the plot itself) or are so effective that they ensure that no one is wants to deal with the headaches involved in divination magic and it gets ignored.
Well, I was reading Full Frontal Nerdity the other day and one of the pages mentioned the idea of "save points" in RPGs. Obviously having anything like a save point is more or less completely out of the spirit of most RPGs...but the idea has some value as a method for divination...
Imagine this. The players bust through a dungeon door and the party barbarian gets filled with poison darts as they trigger the trap on the other side. The rest of the party is horrified to realize that on the other side is a small horde of angry golems which proceed to tear through them. As the last party member falls, beaten to death with their own pelvis, the scene fades out as the cleric recovers from his divination trance and says "guys...I think we should just go through the other door".
Essentially, casting a divination spell allows the group to make a "save point". Then they continue to do what they like, taking any actions they normally would. Once the spell's duration expires (or the group is wiped out) then they must choose whether or not the evens "really happened" or was it just part of the diviner's vision, allowing them to effectively rewind the clock and try again or make different choices.
How long the save point lasts is of course entirely dependent on the power of the spell being used. A relatively weak divination would allow a "rewind" of only a few minutes while more powerful ones might last hours or even days. However, I think it would be far more effective if the duration was measured in real-time rather than game time. A weak divination gives you a 15 minute timer, a stronger one might give you in game hour while an epic spell might let you undo the events of an entire session. This works well because it focuses the effects of the spell on interesting (and chaotic) events even if they might be separated by long stretches of traveling or down-time.
The main benefit of this system is that it would ensure that the effects of divination work well with their intended purpose. The GM doesn't have to try and figure out how the future will go, the future is based on the player's actions just like it always is. And since the events are still determined by random dice rolls the farther the diviner peers into the future the more muddy things become because the "second run" will involve entirely new dice rolls and new results which could lead to dramatically different outcomes.
Of course, this is a good, easy and reliable way to handle divination magic...but is it balanced? Hell no, this may be the most powerful form of divination ever put forward! So here's some suggestions on how I would balance things. The most obvious is of course long casting times, high level (or mp cost) or expensive ingredients but here's some more interesting thoughts:
*Divinations cannot provide specific information. For instance a diviner will (after "rewinding") know that a door is trapped, and trapped with a deadly poison gas...but won't be able to say exactly where the gas came from or where the trigger is. Thus they could assure the party trapsmith that there is a trap but the rogue would still need to make their rolls to find and disarm the hazard like normal. Likewise if a diviner reads an eldritch tome they'll learn that it's a book on demons but they would not be able to recall the actual contents of the book. In cases where this may become important the GM should be vague on specifics until it has been decided whether or not the experience is "real".
*Divinations are one-use only. You can't attempt to create more than one "save" for the same set of events and if you attempt to cast a divination as part of a divination (double-dipping) you go mad from the revelations.
*By peering into the future you are helping to set it in stone. You can make choices to try and make things go different from the vision...but you cannot use any meta-game currency (action points, fate points, etc) to try and alter the events. That means if you had a vision of your party getting beaten up and killed you can no longer use fate points to prevent that fate from happening....
*The GM can of course alter events as well...the visions are visions of the future as they may be, not as they must be. So they players may learn that the exact identity or number of foes or dangers may have changed after rewinding.
Just my random thoughts...I don't know if I'll ever make use of them but they're interesting.