That actually can work, and here's how.
Espers are beings made of almost all energy and they inhabit armor. They can die if their armor is damaged, but if you take a look at how, say, Var works, the weapons can be parts of the body (His dagger, not his spear). Since Espers are very much so made of magic, they are completely magical in nature, and for some Espers (like Var) the magic involves their bodies. If this weapon was originally the body part of an Esper then it wouldn't be ridiculous to say it carries its own magical properties from when the Esper was alive and well. The thing is, Dragoons bodies can't handle magic because of how they work, but Dragoons can still act as a conduit for magic, everything in this world has magic energy with the exception of the monsters. The sword can draw from the wielders energy to use its power.
However, it will have to have a natural limit and there's a few ways this can be done, the most obvious being that once Victors energy is all out it goes back to it's "default" shape. Another limit is the weapons weight will need to change with its mass, the more material the more it weighs. As this is magical by nature it can be less heavy than it "should" be, but there must still be a reasonable limit to where its weight falls. I would also, for balancing reasons, suggest it actually takes time while it "reconfigures" its shape, like you'd imagine something changing shape physically would.
I'd also like to point out the best weapon is clearly the handaxe. <- that's how I say it's my favorite.
, when sheathed and much like inuyasha's sword grow in size when drawn from its sheath. Then transform upon verbal command into any bladed or blunt force weapon in that era. The only limitations it has is the imagination of the one using it. And that it can't split into multiple weapons.
