Some random disjointed thoughts:
Allow rotations to continue through the LB, which may be currently impossible due to some animation like lock coding but I believe such a mechanic could be used for jobs too so may be useful outside of this. By that I mean as you're in your animation lock / casting time you can also use your other skills. An example of this mechanic might be like a dancer like rune mechanic while the rune knight charges their weapon up (using runes while powering up like a Samurai does for their big damage move). Classic general mage example might be channeling magic missile barrage on an enemy while also being able to cast extra missiles by interacting with your abilities. Not that such a feature should be on everything, but may be something new. Especially if they can introduce some other concepts too.. Like cast moving and cast at progress over canceling. By the later I mean lets say Samurai instead of canceling their charge up attack if canceled just released it at a little bit reduced damage (don't even be aggressive on it), such concept for example could be used to better effect on a 'mage bow' class. Rather than their charged up attacks canceling they just fire earlier, meaning the job could still have high mobility but also has more devastating slower attacks (if they were trying to do the sniper like ranger vibe, a bit like FFXI had).
More effects by that I mean perhaps the healing and tank LB should come with damage.. Some steps to gently smooth out the gaps (still leave them, purposefully, but smoother). Which might even mean giving the AoE more damage on the first enemy hit, based on closest to center, and the single target having very middling AoE damage. For tank and healer they can be things in theme like tank LB might deal large radius middling aoe damage when the shields explode (calculated such that 4 player LB isn't awful and 8 player lb isn't better than the actual 'aoe' lb), and healers might be enhanced damage up to X potency concept (again with similar balance pre baked into calculations so 8 isn't OP and 4 isn't useless, alternatively you might just debuff the enemy to make the math easier lol).
Unfortunate nature of the system is there is not too much cooperation... that was a neat part of FFXI (as someone else mentioned). Honestly I'm not sure how well it would actually play out but some sort of interaction would be nice. Of course there could be a cheesy press the button when the other person does fusion-ha like moment to enhance the LB, and maybe that's more than nothing but I can also imagine that infuriating DF and some PF lol. Thought maybe on something of a shared experience perhaps, like you get marked with a shape or color (subtle yet visible) and if you visit your partner you both get a buff like good Dorito (triangle) stack mechanics lol (mechanic for everyone in the party, system will account for dead players and odd numbers and main enmity / tank status.. to prevent encouraging cleaves . . . ).
Certainly a classic concept LBs, in terms of FF in general... it would be nice if they were all unique but that'd also make them a huge massive nightmare lol (unique in effects). Interesting topic, might come back to it later after thinking about it more. As honestly I do normally just see DD use it or go "eh no one will use it so I guess I will for kicks".