Character starting and ending a series with same mindset would be a tad bit boring, especially one of the main characters. It's tempting to say corruption arc would fix things but in Dazai's case that would be running in circles. What's the point of him escaping the mindset mafia installed in him only to fall into something similar again? It'd defeat the purpose.

Instead it'd be interesting if he's pushed into the situation where he has to break the vow he made to Oda in order to actually do good. How so? In order to beat a demon you have to become one yourself - enter Fyodor. Taking into account how Fyodor no longer considers Dazai 'his equal' (or at very least proclaimed to now that he outwitted Dazai) I'd say it's only fair Dazai 'steps up his game'. Meaning starts playing dirty.

Sacrifices need to be made, especially now that he has 'messed up' and ended up empty handed. It's not Dazai's fault that half of ada is dead and Yokohama annihilated, but he might not see it that way. He tasked himself with taking Fyodor down and failed to do that, resulting in all chaos we currently see in manga. Motif of greater good and ends over means is interwoven in the entire manga, so it'll be nice if Dazai is put in that test as well. Let him have some tactile human struggles that mask of 'mysterious and genius' cannot veil. Let him mess up indeed - and let it be ugly.

I think it'll be interesting if Dazai forgoes directly helping people for time being and goes underground in order to catch the rat. I don't mean PM or similar organizations but underground on his own where he might resort to dirty work in order to get things done. Would he like it? Of course not. It'll remind him everything he swore off. But would there be another way? Not really. He'll have to sacrifice the very vow in order to, ironically, uphold it.

When all is said and done, Fyodor brings out the worst in Dazai. I'd love to see some of that 'worst'.