I have a few small ideas I'd like to throw out there since we're still in the "throw it at the wall and see what sticks" phase.
It's cool that Relm can get Interceptor but no sane person would sacrifice Shadow for that, and I also feel like she doesn't have nearly enough use for high Stamina, so what if Starlet Relm got access to Interceptor even if Shadow wasn't dead?
Similarly, I don't think Strago get's enough out of Stamina either, so maybe something like having a higher proc rate on counterattacks with rods? Somebody mentioned having a chance to cast Holy Wind when he covers, that sounded pretty good.
I love the idea of Ifrit Relm being called the Illusionist and her brushes proccing Image in place of their normal procs, and it would probably be the only build I'd ever use for Relm if you put this in.
I think Bismark Terra definitely shouldn't have the Quickcast: Break and Storm, since she barely has reason to use those spells late game to begin with, and DEFINITELY not when she's built for vigor. It'd be a bit much but what if instead she had a chance to auto-Bserk when she Morphs? Terra is an excellent spell-caster even with pure Vigor builds, so this would serve the dual purpose of cranking up her damage for her glass cannon build while also making it more difficult for her to cast powerful damage spells while morphed with that build. For example, I just ran through Kefka's Tower with pure Bismark Terra and she was still hitting 9999 when she was weakness sniping with tier 3 spells.
Another thing that feels like it's a little poorly defined is Terrato Sabin, so why not give it something to exacerbate it's differences with Stray Sabin? I was thinking since one of the main benefits of Stamina over HP for him is that Chakra is stronger, that you could make Terrato Sabin have stronger Mantras and help solidify the HP build's role as a defensive support.
That's pretty much all I've got for now.