The catch: it has to be one, and just one, and you can't make it an overtly WAM moment.

I would put mine in the upcoming This is 50 movie that's apparently in development. They're at their older daughter's graduation from UCLA. Leslie Mann's character has an outfit like this on (even my maxi dress thing aside, she 100% seems like the type of mom that would wear them frequently, which also brings up my next point- I've never met a mom that wears them on occasion-they're either a constant in their wardrobe or they never wear them, my mom is the latter).

Anywho, she gets overheated at graduation (daughter is graduating college, she's getting older, etc) and goes and jumps in a fountain. Fully under, like Cecily Strong with the wine thing on SNL.

The answer to that will largely depend on the type of movie being made. If I had a free hand, it'd be an action-adventure film, and at a point a group of the main characters, mixed gender but inclduing at least three women, one in a boilersuit and wellies, one in combats and army boots, and one in a smart but also practical trouser outfit and practical boots, would have to wade into and through a lake of liquid mud (or possibly chocolate) to reach an objective. This would be done calmly and not under fire or direct threat of attack, so they can walk steadily down into it till it's chest deep, through it, and then up and out at the far side.

The only main concession to it being a WAM scene would be that the camera angles would ensure we could see their legs as they go down into it, probably a back shot as they step down and in, so we can see it engulfing their bottoms, then individual or pair close-ups as they wade chest-deep through it, weapons held up over their heads to keep them dry, and with some plot significant dialog being exchaged, and then a front view as they walk up and out, so we can see their trunks and legs emerging with the goo flowing down their drenched outfits.

They'd then reach the objective, have a battle with the villians, obtain the item they'd come for and flee, ideally down something like a canyon waterslide, which would further soak them while also washing the worst of the muck off their outfits.

I'd put a mud wrestling club as a setting for a random dialogue scene.
No reason given whatsoever, and that would work in any movie set from the twentieth century onward.
Moreover, I would keep the camera on the pit all the time, never cutting to the talking characters, referencing French New Wave as my inspiration for such a daring, unconventional mise-en-scene.
And I'd frigging nail it.

I like Dungeon masters idea, but not the outfits! In mine they would have to be dressed in smart/sexy/formal outfits and heels.

Dinner table scene: agressor disturbs someone at dinner and pushed face into food.
Fight by swimming pool: A bit hackneyed now, - avoid if possible.
The trek across country (escaping?): all sorts of natural obstackles could be encountered.
Shopping: actor spots something for sale and wicked though occurs.

As you can see here are four items straight off the top of my head, it rather depends on what sort of film you want.

Then... it would be a snuff movie as they are all dead.

More necroporno than snuff, it's only snuff if they die on camera during filming. As the old saying (among necromancers) goes, "how hard it is to raise a family depends on how close together they're buried."