Oh god..how about...Memento Mori-ga in a BLM party. It'd only last long enough for 1 or maybe 2 nukes but...+20 MAB x4 or 5 BLM, bahahaha
I have a feeling it'll be on a long-ish timer though, maybe 10 minutes? I'm going to hold off deciding what to get until after I find out recast timers, and how powerful Convergence is. Single-target beefed up Frightful Roar or Enervation could prove to be extremely useful. I wonder if Convergence + Geist Wall would dispel 2 buffs like Fenrir's dispel move?
Edit: Looking at the spell list, there are quite a few spells that were previously just marginally useful which now could become a staple spell to set. Others could be pretty interesting in certain situations.
Diffusion - Exuviation: AoE Erase (and a little bit of HP recovery)
Convergence - Actinic Burst: Single-target buffed up Flash. Possibly longer duration? The blind effect is already pretty potent
Diffusion - Saline Coat: AoE Magic Defense Bonus (+40, decays over time). Could be useful in several HNM fights. Maybe even AV >.>;
Diffusion - Amplification: AoE MAB and MDB +10. Lasts longer than Memento Mori and Saline Coat I believe.
Diffusion - Warm-Up: AoE Accuracy and Evasion bonus. Could be very useful with NIN tank or in melee DD party vs. high evasion mobs
Convergence - Enervation: Single-target Defense and Mag. Defense down. AoE effect is ~-10 Def. and -8 MDB
Convergence - Frost Breath: Single-target Ice damage and Paralyze. Damage is already pretty decent, but I doubt will be affected aside from resists. Possibly higher or more potent Paralyze proc rate?
Diffusion - Zephyr Mantle: AoE Blink, gives 4 shadows. Limited use in melee parties, as AoE that is absorbed by shadows will just wipe this. Could be useful in very specific situations. BLU/THF tend to have this set anyways as it gives cheap Conserve MP.
Convergence - Infrasonics: Single-target Evasion down. AoE effect is ~-20 Evasion. Should be significant.
Convergence - Bad Breath: Single-target debuff + damage. Possible effects typically include Slow, Poison, Silence, Paralyze, Weight, Bind, and Blind. Would need testing, could be interesting. Really sucks as it is
Convergence - Maelstrom: Single-target Water damage. In the mid-60's this was a potent MB spell for a BLU nuke. Could be powerful enough to be worth setting to MB on Darkness or Distortion.
Diffusion - Feather Barrier: AoE Evasion bonus. Very doubtful this would be more useful than Warm-up, but meh.
Convergence - Cold Wave: Single-target powerful Frost. Normally ~18/tick and ~-35 AGI decaying over time. Not great duration normally, maybe longer now, or even higher DoT?
Diffusion - Refueling: AoE Haste. Haste effect is less than that of the RDM/WHM spell and Garuda Hastega (10% vs. 15%), but would be AoE and would allow SMN's to use other buffs.
Convergence - Myseterious Light: Single-target Wind damage + Weight. Possibly high Gravity proc rate?
Diffusion - Cocoon: AoE Defense Bonus. Gives +50% Defense, exceptionally useful for PLD tanks.