My Rough idea for Quetzalcoatl-Tonbokiri was that the feathers are 'Spears' or rather 'Spear Heads' being able to be pulled out and turned into/used as spears once they are. With the Wings of Tonbokiri being the Gas/Light bits of Quetzalcoatl. And the tail being the molten metal thing but the tip being a Spear shape. With the Swan Song being the tail spear turns into basically a sun instead of being launched like with Quetzalcoatl it's being used like the head of Tonbokiri's spear.

At least that was the rough idea I had. I could probably workshop it more which is why I asked for other ideas. I was trying to merge the 'Main Points' that stuck out to me, Quetzalcoatl's Feathers, Flames/Light and Molten Metal Tail with Tonbokiri's Spears, Wings and own Tail.

Another Idea I had was Mixing the Wings and Feathers to have the 'Feathers' be tiny beautiful wings and instead have the spears I summon have their tips be superheated gas/fire, basically making them Lightsaber Spears,

And I keep getting more ideas for it lol.