I think really cool and simple idea that could add a lot of variety would be a Magic vanguard (or even multiple magic vanguards). Right now all the current vanguards do Physical damage and therefore like to stack strength. This plays out the same when all of them get attacked, magic attacks do big damage, and physical attacks do a lot less because of the innate PDEF that comes with stacking strength. If we had a magic vanguard who stacks INT it would add a lot more strategy and variety. Someone running 3 backrow? Use your Magic vanguard! And it would equally balance by being weak to heavy PATK comps. It seems really easy to implement and I think would be a great addition!
Edit: Here's one that I made as an example. Notice the MATK instead of the normal PATK every other vanguard has.
Edit: Here's one that I made as an example. Notice the MATK instead of the normal PATK every other vanguard has.