I'm new to D&D and am playing a 5e campaign as a Paladin with a focus on Dex rather than Str; I am considering multi-classing into Fighter to get the two-weapon fighting style, per some of the suggestions in this answer and associated comments. I also just like the idea of dual-wielding finesse weapons (perhaps a whip and a rapier) per the Rule of Cool.
Thematically, however, I would like my character to be interested in protecting others during combat whenever possible. So I like the idea of adopting the Paladin's protective fighting style, but since that style has no effect without a shield, the choice would be at odds with the dual-wielding I'd like to do.
One option would be to take the Sentinel feat, which is somewhat similar to the protective fighting style, except that it is offensive (providing an opportunity for attacks using a reflex action) rather than defensive.
What I really want is a game mechanic that would allow my character to defend others against incoming attacks without using a shield; to return to the whip example, perhaps this would mean using the whip, Indian Jones style, to interrupt an attacker before they can complete their attack. Ideally, I would like a feat or a fighting style that would do something like this:
When a creature within range of a melee weapon you are wielding attacks a target you can see (other than you), you can use your reaction to impose disadvantage on the attack roll.
This is just the protective fighting style but with a range determined by the weapon used to perform the defensive action and without the requirement for a shield. It also swaps "can see" versus "in range" requirements for the attacker and the target, since the idea is to somehow stop the attacker rather than to thrust a shield in front of the target. In particular, this prevents using the mechanic to do wuxia-style "blocking" of incoming projectiles.
My DM seems pretty amenable to variant rules and homebrew options, but he's also new to D&D, so neither of us have any experience making sure these sorts of options are well balanced and fair, nor do we know what other pitfalls there might be in making up the rules as we go along.
Is my proposed rule variant "broken" or unbalanced in some way? How do I decide whether this should be a new feat or an alternative fighting style? Is there any already-existing mechanic that accomplishes the same goals so that I can avoid having to write my own mechanic?