It is hard to make any recommendations unless we know where you are. As well as the nature of the area being worked.

@sepulvd since we don't know the balance of your current planting design you can always follow your own thought, "maybe add different colors of heather". It would certainly be a safe solution since most of the textures will blend. Monocultures of one genus can be captivating.

There are heathers with gray-silver foliage with white or pink flowers, and also ones with gold to red foliage, particularly during winter, though I no longer remember variety names. A combination of the two looks nice. Erica (heaths) will provide an alternative foliage texture, almost looking like tiny pine branches, but with late winter blooms. I've also planted other ground cover/rock garden plants around the edges of heather beds, including dianthus/pinks, Iberis/perennial candytuft, and some of the low-growing Campanulas.

I use lava rocks in pots so I can water without soil splashing out. It also keeps the squirrels from digging in the pots. I've tried it in beds. It looks nice, but for me it didn't work well. For one thing leaves settle on top of them and when I try to blow them out the lava rocks are so light they tend to get blown around and kicked out of the beds onto the lawn and must be picked up one by one. Hardwood mulch works better for me because the leaves can stay and eventually they both turn into compost. Yes, hardwood mulch, or bark has to be renewed every two or three years, depending on how much it rains and how thick I put it on.