This is interesting but I wonder how this fight is going to end, talk no jutsu failed, secret deus ex machina spell failed, it's going to be hard to deliver a third viable way to end this. I believe the only plot devices left are some kind of sacrifice, the full Hulk/rage way or some kind of godly help, and all them are too bad for such a nice manga. It's been a while since I got so curious about a story.
Maybe Wil doing heroic sacrifice to 'end' Val as a threat (sealing, banishing, putting it to sleep, etc)
It'd fulfill the god of death's prophesized death of Wil, but also ensure the dragon threat is over.
His legacies can be taken up by his comrades.
Menel has sworn to Gracefeel before, so he can be the wandering hero part.
Al and the dwarves can revive their country and form the frontline against demons.
Reystov, the bard girl, etc can continue passing down Wil's tales among humans.
Although one cracked option would be a conversation between Gracefeel and Stagnate
That Wil's soul will return to cycle of reincarnation but when another catastrophe loom over this world his heroic soul will be called upon this world once more....and that this isn't the first time they did this.
i.e. His reincarnation in the beginning wasn't accident but rather a sort of cooperation between Gracefeel (to let his soul reincarnate) and Stagnate (to keep a heroic soul around)