Incompatible Traits

Resonance cares little about what you try to mash together when making or tuning your Echo. That is to say, it doesn’t care if your Echo is a suit of armor and your materials are weapons, staves, etc; it simply merges effects together, for better or worse, and magically accommodates for the missing components in a way that makes sense for your character.
Consider the following: you create an Echo based on a set of armor passed down in your family. You wish to tune it using a flame tongue you recently purchased, and you have enough Chromatite to accomplish this feat. Flame tongue is a sword however, and your Echo is not.
Once your Echo gains flame tongue’s properties, the original sword is destroyed and your Echo gains the ability to set itself ablaze. The 2d6 fire damage granted by this effect is then applied either to a weapon you’re holding, to your unarmed strikes (or natural weapons), or, if you’re so inclined, manifests a blade or blade-like object in your hand for the duration your Echo continues to burn.
Don’t think too hard about it. Make up some cool anime reference and run with it.