Addon support?

[Niwashi]

I don't know if it's wise to block spells from being cast on conditionals, I don't think addons would go that far.. But all the other UI adjustments, well, that's pretty standard for what addons are capable of.

I think being able to customize your hotbars is a pretty standard feature for MMO addon support. Having skills only appear on your hotbars when they're applicable is one of the common ways that's put to use.

For example, in LOTRO, there are a variety of different debuff types that can be cured by different potions, but in the standard UI it requires identifying the type of debuff from its icon in order to choose the right potion type to match it. That can be a bit difficult and error-prone during a fight, so add-ons were created that add the appropriate item-use skills to your hotbar only when you have the matching debuff. That way you know the one that pops up on your screen is the one you need to use. You wouldn't accidentally use one that's not needed because there's no button on your screen for it when it's not needed. (As an additional bonus, that helps clear excess screen clutter while still keeping everything available at need.)

If similar capabilities were offered in FFXIV addons, they'd be able to do things like Limecat suggested, in only offering the Virus skill on your hotbar when neither it nor Antibody are on your target, or only offering Eye for Eye on your hotbar when your target doesn't have that one.