The only time that Serpahinite is going to fuse anytime soon is if someone's like "Quick, we need to fuse to beat a thing" and she's like "what does that mean, I don't even-fine, if it beats the thing, whatever" then fusion happens, and either fight happens or immediate deconstruction, and she'll never say yes again . . until someone gains her trust anyway.

[@Kronshi]
Also, fusions with Sera will be weird. I think most of the time, her weapon won't fuse with the other person's weapon, but their powers, and thus making them more a battle angel with weird wings and what-not. Just a thought to make things easier.