I'd like to get Demonic Beast Dedue, but I doubt he'd be a Rearmed Hero. And we need more armored beasts in general. Nidhoggr makes for our first non-colorless one, but we're still missing blue and green. It does seem likely for Nidhoggr to get additional versions, though, which will probably stay armored beasts.
Admittedly, all Nidhoggrs released will be a ways off, and probably uninterested in an Arcane weapon released before them. Still, I personally don't think of either Rearmed Hero slots or Fallen Hero slots as particularly precious resources. We have pretty much all the relevant kinds of conventional Arcane weapons, after all.