This isn't even about theories for me, it's about that one scene in the Infinity Castle with Douma and Shinobu. People keep talking about a redemption arc or some hidden tragic backstory, but I think they're missing the point. The narrative frames him as this hollow, unfeeling predator who consumes emotions because he has none of his own. Any 'theory' about a secret tragic past feels like fan projection, wanting to complicate a villain who is terrifying precisely because he's simple in his emptiness. The real debate I see heating up is about Shinobu's final act. Was it purely revenge, or a calculated, self-destructive form of protection for Kanao and the others? The manga shows her planning, but the emotion in it... that's what gets people arguing.
As for ships, the Tengen-centric ones dominate the district arc for obvious reasons. Tengen and his wives aren't even a debate; it's canon and everyone loves it. The messy, fun arguments are all about the dynamics between the younger characters. I see a surprising amount of support for Zenitsu and Nezuko, which, after the arc where he protects her box, has some foundation. But the Inosuke and Aoi thing feels shoved in later, like an afterthought to give everyone a pairing. The more interesting, niche discussions are about Gyutaro and Daki's codependency—whether it's a twisted mirror of sibling bonds in the series, or something else entirely. People get so heated over analyzing that relationship, more than any romantic ship from that arc.