In Melbourne the general rule is, as long as you have sunlight coming in during winter, then the more mass the better. Slabs are excellent for providing this mass, especially when insulated. But another idea we use often, especially when energy simulations suggest more mass would be beneficial, is to use reverse recycled brick veneer. This is building a brick skin on the inside of the external walls in place of plaster. This introduces as much mass as required, simply by deciding which rooms and which walls would benefit most.
Now it doesn't have to be brick. Any heavy weight material (blocks, concrete, mud brick, rammed earth, etc.) would do, to increase operational performance. But here's the conundrum. With good design and a concrete slab, you should be able to achieve a 7 star rating reasonably easily (at least in Melbourne) without more mass. Further heavy weight construction, strategically placed, can raise the rating to 8 or 9 stars. But here's the rub. The star rating bands do not represent the same total energy saving in each band. There are much bigger absolute gains going from 4-5 star, than from 8-9 star.
Now, massive materials, especially when fired, or containing cement, often contain more embodied energy to create them, than they are likely to save over the life of the house, when using them to go from one high star to another (say 8 - 9star). If you use virgin bricks or blocks, it is possible, when you also include embodied energy, that you end up using more energy overall, over the life of the house.
This conundrum can be solved by using recycled brick, with cement replaced mortar to keep the embodied energy very low. Non cement stabilised rammed earth, or air dried mud brick would also be a sustainable low embodied energy high mass option.
On the plus side, bricklayers love the idea of laying inside where they are not effected by weather. And walls can either be cleaned down and clear coated to make the brick colour a feature, or bagged and painted to give a more subdued textured effect, perfect for wall lighting to create that mood. Heavyweight walls also block more sound, and create that feeling of solidity many love.