Maybe it's just me, but if you look closely at his entire filmography, I have the feeling that he always alternate a straightforward/grounded in reality film with a fantastical/sci-fi film, or a film with fantastical/sci-fi concepts coming from his mind.
I mean if you take a look, at least starting from his DK trilogy :
-Following(straightforward/grounded in reality)
-Memento(straightforward/grounded in reality)
-Insomnia(straightforward/grounded in reality)
-Batman Begins(straightforward/grounded in reality)
-The Prestige(fantastical film/concepts)
-The Dark Knight(straightforward/grounded in reality)
-The Dark Knight Rises(straightforward/grounded in reality)
-Dunkirk(straightforward/grounded in reality)
-Tenet(sci-fi film/concepts/some sort of synthesis of Inception and his sci-fi concepts/obsession with time)
So again, maybe it's just me, but I have a feeling that his next film will be something more straightforward/realistic, something grounded in reality, something smaller and more personal, like Dunkirk or his earlier films.
I would love a crime thriller with a smaller budget.
Well with Tenet it's the first time he did something with a sci-fi concept, basically all throughout the film, and accepted it to be real. Whereas the sci-fi concept in Inception was still largely in ones head. With a real world somewhere. So I think it's pretty cool that Nolan went a little further with his sci-fi concept in Tenet.
I would love to see something smaller in scale, don't mind what genre or mixture of which genres. I would also love to see another Dunkirk-esque film set in either WWI (or WWII again) or any other huge conflict.
Hell and Gone is always an option, would be interesting to see Nolan do a romance through a historical event. the unknown and debated origin of the fire could be interesting for him to play with structurally/thematically.
I'd be surprised to see him go sci-fi again immediately, my guess is a big horror thriller. If he wants pure cinema, and to continue pushing these intentionally obscured cyphers as characters, then horror would fit like a glove.