On the Knightmare sequence, the intent behind it is muddled. There's an argument that it's meant to be vague but I reject that it's a decision. Way I see it, it needed to be clearer about what it wanted to say.
If it's a tease of a
what if? future of Superman turned evil, I feel the best placement is right at the beginning. Ideally where it ends with Flash running back in time. Basically, X-Men Days of Future Past, Terminator, whatever.
If it's a manifestation of Bruce's fear and paranoia, which is what I took it as, first you drop the Flash and Apokolips stuff and keep it to Superman. Snyder talked about how media is a character in the movie, and what shapes Bruce's opinion on Superman. I'd say the best place then is after Lex's party, when you have the talking head and Charlie Rose talking about Superman's role in the world. Jump between Clark and Bruce watching the same news stories and show the effect on the subject and the audience.
This scene
https://www.youtube.com/watch?v=9RynVyFzcis
Are DC even doing stingers?
Suicide Squad had one, as did Green Lantern if we're going pre-DCEU. Wouldn't be shocked if Wonder Woman had one to tease Justice League. And if Justice League doesn't have one, that could be Snyder not wanting to do one.
What's interesting about Marvel Studios' post-credit scenes is occasionally having them directed by the next director. For instance, Joss Whedon directed the post-credits scene of Thor and James Gunn did Thor 2's. So for all we know James Wan could do an Aquaman teaser for Justice League.