What to do with this?

[gardendilemma]

Any ideas on how I can build this back up with minimal effort and expense? A few years ago I built a stepped bed over a filled-in pond. However, a lot of the plants are now past their best and some of the edging at the front has rotted away (very old, not part of the new bed) but the rest is solid. I have taken away the dead / rotten stuff and would like to edge the front so it is slightly higher but obviously now have a metre long space to fill before I start. Was thinking a tall planter but not sure if it would look weird. Is a rental property so don't want to start creating an expensive project, just want it to look tidy.

Thanks,

would like to edge the front so it is slightly higher but obviously now have a metre long space to fill before I start.
You seem to have attempted to echo the vertical effect of the edging. Would it work to think of working horizontally instead? - use the rocks on the flat to form a rockery that tumbles down from the heights to path-level.
Hide the awkward bits with lots of ground cover and trailing plants.

What about planting campanula it’s really low maintenance and looks lovely cascading over rocks and edges.

Thank you both, have had a play about with the rocks and that definitely looks the way to go @senua. Will look out for some campanula when I get round to plant shopping too.