Let's write this here out of the list since it's my personal fave:

Darkness abilities should be Stat Dependant, like Vayne SBs, and merge the similiar ones.

It's the only ability school that applies to both PHY and MAG guys so it would make sense to reduce them this way.

You could craft "Dark Scythe" once and use it on all Darkness characters, be it Seifer or Golbez.

I have both Zone and Scythe but this way I could mix teams with 2phy Jecht+Gabranth or 2mag Raines+Seymour instead of always Jecht+Raines because 2 skills.

It would also grant mages to use a double-style with Sanguine Cross or a special version of Dark Bargain for magic.

I'd also stop to put Memento Mori on Jecht by mistake XD