Before I start, this doesn't mean I'd be expecting such scenes to be filmed. Quite the opposite, in fact. Certain technical realities, such as performer safety being #1, make most of not all of the ideas here pretty well unfilmable. Artists and animators, though... go for it!
Burial is top of the list of sinking substitutes, I believe, as it does involve coverage, just of a different sort. Beach burials have been popular around here, and I was impressed by the sand box of DIDVP fame - lady locked in box and slowly buried in "sand" like substance. An hourglass trap is another classic of these. These work for me because of the coverage idea... not quite as cool as being consumed by wet earth, of course, but a fair substitute.
Dry sand sinkage is great too. One can substitute anything dry and granular... grain being one of the more likely ones.
Wet cement is a cool visual too... whether it is poured on the victim, slowly burying her, or set up as a bottomless pit of wet cement for the victim to wander into, or be placed in, to sink into. (Although I'm sure there's something in the building codes against that...
Sinking in pudding, syrup, honey, chocolate etc. would all fit into these categories as fair substitutes too.
I have to mention blob vore in this. Some think of it as "intelligent quicksand" in the sense it actively pursues its prey, yet consumes it as if it was sinking helplessly. Needless to say, I love it.
Anyway, that's my rather long winded two cents. Anyone else have thoughts on this?