A bullet-like spell that can bounce off of walls. You'd be able to do some nice trick-shots with it, but unlike most spells, it would be a pretty small projectile, so you'd have to be very accurate and it'd be hard to hit small targets like Bats with. It'd make a good Earth or Light spell. You could make multiple element versions of this, with one being rapid-fire spammable, and another being a single, very powerful "Sniper" shot.
A constant-damage spell that keeps going as long as you hold down the button. It's damage could start slowly increasing if you hold it on the same target for a few seconds. This could be anything from a Water or Lightning steam to a Misasma or Flame thrower. Obviously, this would be a good single-target DPS spell, best against larger (Easier to keep the beam on) enemies and fare poorly against crowds.
A Damage-Over-Time spell. When you hit a target with it, they take very low damage, but a damage-over-time effect is applied that does respectable damage. With each hit, the damage-per-second and duration of the DoT increases (Capping out after about 5 or so hits). To get the full effect, you'd need to let the DoT work instead of just spamming it. It would be useful in rooms of enemies, where you could DoT up multiple enemies, then dodge them until they die. It'd be less effective against single opponents or opponents that you need to kill quickly. This would be a nice spell for Fire or Miasma.
AoE explosion spells. They could be either Explode-On-Impact (Exploding like a rocket launcher hit) or Explode-at-a-set-distance (Like a Flak cannon) These would have only average DPS, but have the advantage of being able to hit multiple clumped-together targets without having to get them to "Line up" like the current piercing spells. Fire, Water, or Lightning would be good elements for this.
Honestly, Fire needs the new spells the worst. It has no non-knockback spells in it's arsenal that I know of, it's projectiles are generally slow, and the damage is sub-par. I'd at least like to see that "Constant Damage" spell above made into a really sweet Flamethrower.
Also...try to have some non-knockback spells. Because honestly, having to keep correcting your aim because your spells knock Espers around like ragdolls is more annoying than helpful and we already have enough "Splashback" type spells.