[Autumn]

Spellthief hasn't gotten a lot of new DLC in a little while and has been kinda left in the dust, I think introducing a few new-ish monster spells to its possible spell repetoire could make things more interesting for them, not every single one of them has to make it in of course, and possibly you could place restrictions on them for players specifically which I'll also mention, but here's what I'd possibly consider making stealable for spells:

Ominous Spell (Changed to magic)- Currently heals for 500 HP and provides LV/3 as all bonus stats to the Ice Giant that casts it after a set amount of turns
-If a player uses this ability, the LV is capped to 10 EDIT: the heal is halved as well.

(I also just love the idea of players being able to turn HUGE after the cast goes off, this is countered entirely by a single null like requinite or shells or wash away etc.)

Self-detonate - When adjacent to an enemy, destroys self to deal Fire magic damage to all within a 3 Range circle equal to level + 500% of Fire ATK. Requires a certain number of turns to pass before it can be used.
-If a player has this ability then a 5 round cooldown will be put into place at the start of the match, yes it would kill the player too, possibly have damage cap out at player LV*10 or current health (whichever is lower) so you don't reach scenarios where this deals over 600 damage.
(Please let me chiaotzu someone.)

Chilling Howl - Unleash a freezing roar. Enemies in a 2 Range circle take Ice magic damage equal to 150% Ice ATK It can also inflict Frostbite LV X (X = 10 or LV/2, whichever is higher) for 3 rounds.
-100% SWA scaling should be added so it does decent damage

Banshee Wail - Unleash an ear-piercing scream. Enemies in a 2 Range circle take Sound magic damage equal to 150% Sound ATK. It can also inflict Fear for 3 rounds.
-100% SWA scaling could be added for more damage, Domino Resonate already does this better so may as well.

Denile - Fires its own jam at an enemy within 3 Range who is enchanted with an elemental, dealing Water magic damage to them. Also seals Elemental Impacts and certain elemental enchant effects for 3 rounds.

Howl And Haunt - Roar at a target within 4 Range, inflicting them with Hunted LV X (X = 10+25% of level) and gaining Quickness LV 5 for 3 rounds.

Demon Drool - Spits a vile, demonic acid towards the target in a line. The first enemy hit by the projectile takes Dark and Water damage. It can also Poison the target randomly.

Immolate - Changed to magic.

Angler Light - Changed to magic.

Thunderclap - Changed to magic, FP cost lowered to user's LV/2 (currently equal to LV)

Railgan - Allowed to be stolen from Beetles.

Anything I didn't comment on specifically is probably more than fine for players to have access to, I just want to have a blue mage plz