Looking for help on a new FF5 ROM hack

- All elemental damage, etc will be one of the four elements in the story (Earth, Fire, Wind, Water)
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- Some of them cannot use certain jobs
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- Equipment overall is altered (a good 40% of it) with a variety of things like Bells being strong versus Undead

Some of these are really neat ideas.  I like tying elements to the crystals.  That'll also let you use whatever elements weren't covered by the spells for special effects, like bonus damage for weapon types.  For instance, making all spears Lit element and making certain enemies weak to Lit lets you set up a spear bonus.
...Though FF5 might have mechanics for that already; I don't know anything about its battle engine (or at all, really).

Restricting jobs is also a neat idea, and could help with some of the redundant 'fighter' type jobs.  I can picture restrictions like "Only Galuf can use Monk" and "Only Faris can use Dragoon".  It might make the game a little linear in the beginning, though, with so few jobs on hand, since you're talking maybe 1 job per character.  I suppose it'd depend on how much overlap you allow.

The thing about bells and the undead is a really cool concept.  I hope you explore other ideas like this, like a bonus against plants for axes and a bonus against mundane animals for whips.

Can't really help you on the ASM front; what little custom code I made for my hack was hexxed directly, and was heavily contingent on the extensive disassembly groundwork of Assassin and Novalia Spirit.  I think I saw somewhere that Lenophis was doing disassembly for FF5, but I haven't looked into it.  Everything is also making a custom browser editor for all the 2D Final Fantasies, including FF5 I think.  If you aren't already using it, maybe it can help with your goals.
https://everything8215.github.io/ff6tools/ff6tools.html