The next caster

[ReiMakoto]

Mechanically speaking a caster is a DPS that has to turret in a game that encourages movement mechanics. To combat mechanics, each caster has a unique set of abilities to allow them to move to safety:
BLM has rotational instant casts and teleports
RDM has dualcast
SMN has trances / will have egi assaults

A fourth caster would desire a unique way to engage with this core mechanic of casters unique to it. If one were to go Time Mage for example, self applied haste might be too close to Lightspeed or Triplecast, but it is indeed one way to go about it. Perhaps a reverse Lightspeed; the gcd will do double damage at no cast time, but have double GCD. This comes close to treading on Dualcast, but it's another possibility.

To try and think of classic time mage, warping around with a shukuchi style ability would fit time mage well, considering in games like FFT they could literally teleport everywhere instead of walking, combine that with a ruin 2 esque ability and give the teleport a short cd and there you have instant movement, you could even give it a rewind esq ability. If you want to increase skill ceiling a bit you could even add a "Time Loop" style mechanic for their movement, where you activate the buff and it will store a copy of spells cast during its duration, then when you need to move you can activate a stance that makes a copy of you "from the past" stay and cast the set of spells until you next take an action or it reaches the end of its sequence.

Self Applied haste doesnt need to act like triple or light speed either, it could be the GL4 of casters, reduced gcd and cast time and not instant casts