Does Protect really need to have a restriction to only basic attacks? Surely needing to be cardinally adjacent is enough of a limitation already. I would say it could be safely expanded to work against all single target attacks, considering there are a number of ways around it's limitations still. Charm, in addition, basically already works this way as far as attack intervention goes.
Buuuut if this was my only suggestion, this thread would probably belong in Balance Fu. I think giving such a buff in the types of attacks that can proc protect would be enough, but here are some alternatives.
This could work as a soldier passive or a chivalry subtalent. It could perhaps have multiple levels that cause the skill to affect more types of status effects.Shelter the Weak - If an adjacent ally is afflicted with Knockdown, Blind, or Fear, your Protect skill will defend them against any attack targeting them. Not just basic attacks (Or skills that perform them).
Modifying protect could be all the archetype of the 'Shield Defender' really needs. But a brand new soldier skill with a more powerful version of the protect effect in exchange for some momentum and FP could also provide the desired flavor. Perhaps at Max Rank, it could block the next two instances of damage instead.Intervention - Shield Skill. Target an ally within X range and rush to their side to defend them, causing the next damaging attack or spell that would target them to target you instead.
Shoutout to Spoop's 'King's Guard' skill suggestion over here, which also would service this 'Protect Others' archetype. viewtopic.php?f=8&t=6883