Gold foliage favorites:

Yucca 'Color Guard' - these can take part shade surprisingly well.
Dicentra 'Gold Heart'
Aralia 'Sun King'
Hosta 'Dancing Queen'
Nepeta 'Limelight'

What about Hakonechloa macra âÂÂAureolaâ ? I was never keen on yellow-foliaged plants until I saw this combination in a garden on the local garden tour a few years ago:
{{gwi:149049}}

I'm currently in the process of trying to make a 'golden path' along one section of path leading under the white pines in the garden. The golden forest grass is a significant part of that, along with bright green hostas (e.g. 'Sum and Substance') and ones with yellow variegation. This is a very bad picture but shows pots the grass with a 'Frances Williams' hosta on the patio:
{{gwi:202056}}

Would that grass work for you?

Why do you have problems with heucheras?

Like I mentioned, nothing grassy in this area because there is already sedge and Siberian Iris.

I'm done with heucheras - they always peter out on me, I'm tired of replacing them.

Tradescantia 'Sweet Kate' but it has elongated foliage.

There are some St John's Worts with golden foliage like 'Brigadoon'.

Enough sun for some of the yellow euphorbias? I have read some can take some shade.

My coralbells are frustrating me this spring. They all die out in the center, even if they are not heaved up and they are still little.

I think the Hosta would be a good choice-- there are so many to choose from.

I would recommend a variegated Iris, but since you already have Irises- you might not want more sword shapes.

And there is also sunrise euonymus fortune-- but it likes to spread and so you have to chop it off all the time. Never seems to mind tho.

Leucosceptrum japonicum 'Gold Angel' goes by the common name of Japanese Shrub Mint, although it doesn't ramble like culinary mints and despite its size (3'x3') isn't a shrub. It has gold to chartreuse foliage and negligible flowers which come too late in the fall to appear for me.

Clematis alpina 'Stolwijk Gold' would like at least 4 or 5 hours of sun if you want blooms, but doesn't need full sun to produce its lovely chartreuse foliage. Not a great photo, but it really does look gold compared to nearby plants. Flowers are sort of a steely blue, less mauve than the photo seems to show.
{{gwi:208569}}From June 5, 2012

Hosta 'Gold Edger' 'Heavenly Tiara' 'Grand Tiara' 'Lemon Lime' the list of hostas that are all yellow or yellow variegated is pretty long....

Geranium 'Ann Folkard'

Dicentra 'Spring Gold' or 'Gold Heart'

Golden creeping Jenny though this can be aggressive

Scotch Moss
Sagina subulata 'Aurea'

Centaurea 'Gold Bullion' perennial bachelor's buttons

Bergenia 'Lunar Glow'

Brunnera 'Diane's Gold'

Corydalis 'Berry Exciting'

A word on Ann Folkard: I also live in zone 5a/6, tried AF twice (at least 4 plants in different gardening seasons) and she did not survive. Although word is she's hardy here, she didn't get the message and just peters out in two seasons.