I don't post new ideas very often ever since they took my treasure map idea out of WoM. Now that we have a new game to look forward to I wanted to put up a few spell ideas that I think would work very well with the new direction. Hopefully there is still room to take really good ideas into consideration.
Curses. Now, most people don't understand the function of curses, so I will educate. A curse is not something you simply cast on your enemy to weaken him or her. That would be only one type of curse. There are more uses, such as cursing your own units to win a vital battle, knowing that in the end the curse will drive them mad or cripple them in future battles. I suggest a demonic set of spell books that use such magic to increase strength for a single battle and then permanently weaken the target in strength and constitution. The idea is to sacrifice the long term in return for an immediate result.
Another use of curses could be in the Sovereign creation section. Choosing a weakness is rarely going to add anything to the game. However, choosing a powerful ability that only your Sovereign can use could potentially rewrite the entire power dynamic during a game. A common ability would be to be able to command any units that reach within one tile of your Sovereign in battle. This would force your enemies to avoid you in combat, though you would be their ultimate goal. The Sovereign would then become much like the chess piece, except he would be able to make any creature his pawn. The drawback of choosing such a power would be higher recruitment costs for any hero, as they would fear becoming one of your drones. There are many such curses and powers I can think of that would really make this game stand apart from its magical predecessors, specifically WoM.
Additionally why not have a few very rare creatures that have specific abilities to sit beside our mages in battle? That was one thing I liked about the 1.1 version. A serpent that can poison, a tree that can entangle, a wraith that can cause fear, a demon that can posses, an assassin that can have infinite moves once per battle. These are all hopes of making unique abilities and thereby units. All we have is hope at this point.
If the spells are easier to create than the previous game, I will try to make a few good spell books at least. If the Sov creator is moddable, I will try to add curses to the system, though I would think that is one of the parts of FE that will change the most.