The current passives are hit-or-miss in most cases; prime example being Raven Tail's "Teleporting" nonsense. To be honest, I think the main passives need some changing so that 1) they don't offer unfair advantages to those who already have a certain buff naturally and 2) don't feel like they should be naturally cast-able spells.
That said, my first point is that passives should affect known spells and/or statistics, or they're not really 'passives', just unique near-uncontrollable spells. To that extent, I believe passives should help speed up stat leveling when completing missions, and have bonuses that help gain/lose GS depending on how the Standings look across all guilds.
Fairy Tail -- "Friendship" passive; When members complete missions that require the killing of NPCs, they receive an additional 5 points of Level EXP if their GS is above 200. Speed, MP, and Str receive 10% of the required exp.
Sabre Tooth -- "The Strongest" passive; Gain 10% more Str, Vit, Def exp from training when GS is above 250. Mission completions award 10% of the required stat exp to Str, Vit, Def.
Raven Tail -- "Dark Guild Legacy" passive; when members complete missions that require killing players, GS earned is tripled if GS is below 300. MP, MR, Vit receive 10% of their required EXP.
Blue Pegasus -- "Parfum" passive; Receives an additional point of GS on all mission completions, and 10% extra jewel, as long as GS is above 200 and below 300. Vit, MR, and MP receive 15% of their required EXP.
Lamia Scale -- "Disadvantaged Reversal" passive; if your GS is lower than another guild's, all mission completions grant an extra 2 GS, otherwise they grant +0.5 GS per completion. Vit, Def, MR receive 10% of their required EXP from mission completions.
Quattro Cerberus -- "Wild FOOOOUR!" passive; When another guild completes a mission, the next completed mission by QC members will grant 4 GS regardless of rank as long as your own GS is below 300. Str, Speed, Vit receive 10% of their required EXP upon mission completions.
Mermaid Heel -- "Stunning Beauty" passive; Regardless of your guild's standing, all missions give 1 GS if they are lower than B-ranked missions, and all missions will give an extra 15% Jewel as long as your GS is below 300. MP, Def, MR gain 10% of their required EXP upon mission completion.
Just a thought. XD