Plotting help for isekai/dungeon story?
I've pm'd you earlier regarding my offer.
But, I think the "writer's block" (if you can call it that) that you're facing right now may be caused by limiting yourself to a certain cool 'thing' that you want to see in a story. What does this mean? This means that you have not fully thought out the story that you want to tell.
A personal suggestion of mine would be to consider a message that you want to tell in the story and base your story around that message.
For example, 'Batman: the Killing Joke' very much has a message around the character called the 'Joker'. The message was that a single unpleasant day can turn a normal person into a monster. And the story does base itself around this message very well. It sees the atrocities that the Joker has caused in the movie and explains what caused him to do such thing. Of course, you don't have to anchor yourself into one single message, but identifying the main message you want to tell should be helpful in fleshing out your story.
Just a personal tip from me.
I agree, theme is one good way to approach that. I do have some themes vaguely in mind, but I haven't put them into words yet, so I should work on that.