re: spells, I'd be interested in a spell 'forge' where you craft new spells. Something players can do to create new custom content.
Spells break down into two components:
1) Special Effects (audial/visual)
2) Mechanics (gameplay and ruleset impact)
(note to self, and to others... did I miss anything?)
Special Effects:
Each of the 5 schools may have a common or set of common effects to represent the audio and video component. A player may then overlay additional effects to provide sight/sound variations between spells of the same school. As a simplistic example, perhaps all Fire school spells have a reddish video component and an animation that is reminiscent of fire or smoke - and a base sound loop of some bonfire or other. But a player may then overlay addition color or sound effects to provide some unique flair for his/her newly 'forged' spell (Atomic Fire Bunny - Rank I).
Mechanics:
In many games (or at least those that I have experience with) spells - despite seemingly endless variety - spells/skills/feats/etc. tend to be fairly limited in possible impact. A fireball spell may do an initial amount of damage over a radius, and targets may then experience damage over time effects, with the damage being of 'fire' type (to provide for any resistances or mitigations the targets may have). Lightning bolt spells are very different thematically speaking - but they still deal damange. The effect is essentially the same type of spell (damage dealing) though perhaps the two spells vary in their degree of damage and damage over time impact to a target.
I make this point to provide a conceptual foundation for the idea that the game's 'spell forge' (or out-of-game toolkit) contains a pool of pre-created spell effects. These spell effects are assigned to spells as desired. So, if someone wants to create the above example (Atomic Fire Bunny - Rank 1), we've covered the fluff of animation and sound, but now we want to determine what kind of in-game rule or mechanic is impacted by this spell. In our spell forge, we assign to our new spell:
* a damage type - useful to determine what, if any, target damage resistances or mitigations come into play
* a damage variable or possible range of damage (conceptually, we can call this 'hit points of damage')
* a range variable - is it a melee or ranged attack (or some other range related mechanic)?
* damage over time value?
* non-damage effects? (e.g., modifies target movement rate, to hit combat chances, adds a poison effect, reduces morale, etc.)
* required resources - do you need gold, mana, some number of widgets to research and/or use the spell?
There aren't that many ways to affect a target of a spell - as long as those are defined and exposed to our theoretical spell forge, we can create a fair amount of customization and variation for the player community.