When my watercolor paintings need a little "umpf" it's time to break out the farm fresh eggs and stir up a little egg tempera. An almost impermeable substance, egg tempera can add a lot of dimension to a watercolor. Purely speaking, it's not for use on paper- it's more appropriate for sturdy backings like wood. But if you're using massively thick Arches 300 gram paper, it may be used in moderation without causing worry.
As an oil painter who uses paint to build up surfaces and then alter them over and over, adding tempera layers to a watercolor can bring the medium closer to what I prefer. I love seeing light, spontaneous watercolor paintings, ones that showcase transparent effects and ways to use the white of the paper. But I usually revert to my old ways of layering, and so constantly seek for ways to add substance to the paint. Every artist has their tricks, and this is just one of them. I remember seeing Andrew Wyeth watercolors at an exhibition that were heavily worked, scraped and layered and feeling a kindred spirit.
Building up an image bit by bit, paint stroke by stroke, helps to make the painting feel like more than just a flat surface. I'm trying to bring a luminosity to the image, and a past that the upper final layers have to be viewed though.