Hosta, ditch lilies do well but a bit taller. Ajuga is pretty tough too and lamiums can do whatever they like depending on the variety.

The Ajuga would look nice. Some have burgundy leaves with purple flowers.

How much sun does the area get?

A lot of ferns like wet feet, and will even take sun if they're kept moist enough.

Fernleaf Bleeding Heart. Grows about 6 inches tall. You can get pink or white flowers but the pinks are more vigorous. Flowers all summer.

Trying to get rid of all the ditch lilies. Hostas didn't make it there the year before. I have Ajuga on the other side of the lava rock fountain, grew so well I may have to divide. The Astilbe seemed to do well.
The area is part sun in the hottest time of the summer.

Thanks for the memory jog Tiffy, forgot about Dicentras
I had them for years, they faded out and I didn't replace
Now envisioning some taller ferns for the top, with Astilbe and the Bleeding Hearts.