Sounds like it’s a really challenging site, because of very low light. A sort of Japanese-ish arrangement of moss and stone that you’re hinting at might work, but if ferns and impatiens have given up the ghost so fast, most other things will probably do likewise. I have used the toughest of houseplants, like mother-in-law’s tongue, Sansevieria, and some of the large-leaf plants so commonly used in dark offices, to "dress up" a dark spot for a few months…sort of using houseplants as annuals. But your use of the term "grotto" makes me thing not much of a permanent nature nor any flowering annual will survive there. One more idea: I have also used things like birch branches ("arranged" in big metal containers like giant floral arrangements) to brighten dark areas and make them look garden-y. I know this is not horticulture…but sometimes the site’s not conducive.