Both are fairly commonly used 5* shields, both resist shock, fire and their respective damage types of shadow and elemental. There is no piercing-resisting variant of either of them. That's one side of the issue, the other being that the Barbarous Thorn Shield, which offers less defence for a sword damage boost, is considered almost the same thing but better.
Therefore, I propose lowering Aegis' piercing defence (but leaving the Normal alone) and giving it fire/shock resist. This would help separate the BTS and Aegis, and make the both different, entirely viable choices, and fill out the missing spot between the CoA and the GOS
You can't throw up the argument that Aegis is part of the Spiral Order gear, and therefore shouldn't be specialised - it already is. It resists piercing. Lowering that and giving it a slight boost in a different specialisation that would make it a bit more widely used can't be a bad thing. You also can't say that the Azure Guardian gear would also have to be buffed to match - though it would be nice, the Almirian Crusader gear doesn't match the Crest of Almire. It resists curse instead of shock and is weak to fire.
As to how it'd be introduced into the crafting line, I say don't! Leave it out altogether until you hit the 5* shield, because otherwise it's even more of a sharp turn between the Mighty Defender and the Heater Shield, plus it matches the setup that the Blackened Crest has, where the status resists are only introduced after you upgrade it from 4* to 5*
I like this idea a lot. I've gotten kind of sick of the constant use of the BTS over every other shield.