Murder mystery and 5e reminds me of one of my DM friends coming up with increasingly large numbers of reasons why the murder victims could provide no useful information whatsoever to our player that had speak with dead
Have you tried making just a big roll table for random rooms/encounters that would be thematically fitting for your mega dungeon?
My party didn't actually have Speak With Dead when they went into this debacle, but I'm only just realizing that it wouldn't actually have made that much of a difference anyway as there were never any (intact) bodies to be found...
They
did spend most of a session interrogating and torturing a local school of fish though, and while the pitiful piscines naturally couldn't provide much help in the way of information about the land-village nearby, they did at least manage to point the players to a "human fin(ger)" that had fallen into the stream a while back, and the identifying wedding band thereon.
As far as encounters and theme goes, I'm mostly solid on that front... The issue comes in the form of the layout, and the layout
is important because I'm specifically putting them into an old-school-inspired dungeon crawl, mapping and all.
And therein lies the issues I'm giving myself with random map generators: I feel that the collections of rooms are too chaotic and disorganized for being the ruins of a lived-in civilization of dwarves, and that there should be some more structure and intention behind the areas and interconnections... ...which of course means more doing stuff by hand, and planning out streets and districts in my head, which I'm historically
absolute balls at.
Another issue is that... Well, nice and orderly districts aren't necessarily the most interesting to map and venture through. There, the more wild and interwoven randomly-generated dungeon rooms can be more compelling as they offer more varied and unpredictable environments. So I'm trying desperately to come to some sort of compromise between the two extremes, and by and large I've just been procrastinating instead of actually laying down rooms and tiles, because apparently I've just got some colossal mental block when it comes to
actually doing things. Might potentially be related to my never having designed dungeons before.