Mud Spikes, Wind Spikes, and Aqua Spikes
Seeing as we have spikes spells for all the other elements:
Blaze Spikes (Fire)
Ice Spikes (Ice) - Can cause Paralyze
Shock Spikes (Lightning) - Can cause Stun
Dread Spikes (Dark) - Damage taken becomes HP gained
Reprisal (Light) - Shield use increased
Why not add in Spikes spells for the remaining 3 elements?
Mud Spikes (Earth) - Can cause Slow
Wind Spikes (Wind) - Can cause Gravity or Silence (Silence might be overpowerful, so I offer gravity as an alternative as it is also wind based)
Aqua Spikes (Water) - Can cause Poison
The idea for these new spells is relatively simple and would be no more powerful than the current spikes spells. I've personally always found it silly that there weren't spells of these elements in spike format. While, every other spell type in the game generally does have at least one spell of each element.
As for what jobs that would be able to use them? Naturally, it would be Black Mage and Red Mage, like all the other original spikes spells (not dread spikes or reprisal which are drk and pld respectively).