The Gallardia was in front of and sort of "underneath" a butterfly bush,.

And so I take it that your "butterfly bush" returned for you this spring?

Are you sure these plants aren't still alive? I don't know these particular plants at all, but it's too early even here to declare some things dead...

There isn't a Gaillardia named Peach Melba that I know of. Did you perhaps mean Heuchera Peach Melba? Or the peachy colored Gaillardia 'Arizona Apricot'?

If it is the coral-bell you will definitely know if it is dead or not. I've found 'Marmalade' to be a much longer lived coral-bell than most in the orange-y shades.

If it was a Gaillardia you may try a Coreopsis (tickseed). Lots of yellow varieties- 'Nana' is a nice dwarf one. The coppery 'Sienna Sunset' is also great if you needed something in that shade.

I don't grow acanthus either, but I would give it a bit more time. It depends largely on the species planted too; some are hardier than others. If it is dead, Baptisia (false indigo) may be a replacement option because it has "spiky" like flowers.

I'm sure others will have more ideas too ;-)
CMK

D'oh! It's Gallo Peach, not Peach Melba.

The butterfly bushes are fine--in fact, they never completely lost all their green leaves even in the most brutal part of winter.

Coreopsis might work.

Baptisia is beautiful but I wonder if it's too big? Acanthus sends up a narrower bunch of spikes almost like foxglove. Baptisia looks pretty bushy, and it's going to be next to a hydrangea bush.