When I refer to "the spell's power" in the following suggestions, I assume at least one of the following is true: 1.) the spell's power is directly tied to the pony's level, 2.) the spell's power is directly tied to the pony's Magic stat, 3.) the spell levels up with repeated use over time, 4.) the spell will be available in multiple tiers (e.g. Final Fantasy's -ra, -ga, -ja).
Teleport is an obvious one. It can help you flee battle more easily, as well as be used remove enemies of sufficiently low level relative to the spell's power, forgoing any EXP and items you might get from defeating it.
Levitation is another. The object that the spell picks up and throws changes depending on the area you're in, so it's sort of like using the Geomancer class from Final Fantasy V. For example, you can throw a swarm of leaves in Everfree Forest, and gobs of swamp goo in Froggy Bottom Bog. Additionally, as the spell's power increases, the objects the spell throws get bigger or more numerous.
Illusion increases your chance to evade enemy attacks by casting a mirage of yourself. As the spell's power increases, the spell can boost evasion rate further, or be spread to multiple ponies.
Firecracker is an attack spell which shoots out a colorful flame which explodes on contact with a single enemy and deals Fire damage. As the spell's power increases, its damage increases, and it can eventually target multiple enemies.
Lightning is an attack spell which conjures a thundercloud that strikes a single enemy with a bolt of lightning, dealing Electric damage. As the spell's power increases, its damage increases, and it can eventually target multiple enemies.
That's all I can think of at the moment. I'll comment again if I have more.