Single woman starts working in a nursing home and takes care of old people ( men's and woman's ). She got blackmailed and now needs to also serve them in another way ;)
as extra plot :
1. Her old mom and dad also joins this house
2. They told them to bring her daughter to job
It'll likely be popular with the rape/blackmail/nonconsensual fans, yet there's really no reason to interject those elements, as the concept would work as well if it's ALL consensual. That way, you could add the occasional nonconsensual element, yet it be a side plot, not the main story idea. That way, there's more areas to explore and the story is less limited in scope.
Think about it, she starts working, seeing how those still sexually active are frustrated, takes pity on one, they both enjoy it, word starts to spread and … all manner of complications and shenanigans ensue.
Another point, in any old-folks home, nearly everyone has their own kids, so suggesting someone sacrifice their own would likely turn out badly. It's the same reason why pedophiles don't survive long in prison, as even the worst criminals can't stand the idea of them remaining there, imagining what they may do if they're ever released.
The elements are all the same, there are just more options to choose from and explore, without detracting from the core concept.
Though, the idea of old folks having sex is a turn-off for many, who prefer thinking of young and hot hotties. Which is why, it's more common to interject family into a story, so you're adding different types, rather than all old folk. A few aren't bad, yet everyone being 'over the hill' with only a single hottie …
My imagination is already purring over the options.