They could just have it so that the weapons "evolve" because of story related reasons or because of experience/upgrade points. DmC Rebellion had concept art of a devil side and angel side cross guard, similar to DMC Rebellion's human skull side and demonic/horned skull side cross guard. Ninja Theory probably wanted Dante and his weapons/powers to evolve a bit more but was cut by time, technology, etc. Heck, there were Devil Trigger transition concept art of Dante progressively becoming more and more demonic with his Devil Trigger and having Angel/Demon mode being more pronounced with each stage. But alas, time and technology may have prevented them from implementing this; all we got were flashing eyes near his portrait and symbol on his back.
If anything, Rebellion could form a new cross guard. Eryx could form greaves or become claws, two bucklers, be able to project the flame fists always/more often. Aquilla could have more copies of itself while thrown, split into dual blades, be used more like chakrams (physically attacking and not throwing), or whatever else that could happen. The Arbiter could shrink and split into two axes, have the blade of the axe constantly spinning so during impact it will hit multiple times, allow Dante to emit phantoms more often for multiple hits, morph between a mace, warhammer, or flail. Osiris could similarly split into two kama-tonfa-like weapons, gain a blade at the end, have a more flexible blade and can switch between a halberd/spear or scythe, chains attached to them while splitting into two, become a kusarigama, allow him to use wind related abilities (explain to me the weird tornadoes that pop up during Rake and and Raze), or whatever else.
These evolutions could just lead to different weapons besides ones from gained by new bosses.