How about changing armour effects to be dual class? Or maybe adding new dual class armour sets?

It's not a bad idea. The Frost and Forbidden armors are some of my favorites, but there's not enough armor sets in the game, so changing one to dual class would take away from its functionality as a mono class set and leave the class with nothing else to fall back on. I also don't want to introduce new items for the sake of keeping the vanilla feel. Making dual class accessories might be more doable, though, since there's a lot more of them.

There's definitely potential for some accessories to be dual-class, I think? The Magic Quiver and its upgrades, and the Old One's Army accesssories are what comes to mind.

For actual weapon suggestions, what about the Onyx Blaster gaining a Magic dual class? I feel like the special dark crystal projectile alongside the use of a Dark Shard to craft it could sufficiently count it as magical enough.

I think I prefer the Onyx Blaster as just ranged considering the basic shotgun blast is still a major feature of the weapon. The onyx projectile also explodes, which is more of a ranged trait, rather than pierces, which would be a magic trait. Not that that's a hard rule, but hopefully it helps explain my thought process.

Maybe Spirit Flame could be changed to Magic/Summon, since not only it "summons" slow homing projectiles but it would also give this weapon more of a niche compared to the more easily obtainable pre-mech magic weapons.

Oh yeah, I completely forgot about Spirit Flame. Good catch.