So we have fire, water, earth (seismic), and air (electricity) blast sets. You may be able to achieve all 4 elements, if you include certain epic pools and incarnate powers, but not in just a single set. I was doing some thinking, and there are basically 2 ways to achieve such a mixed set:
1. This is the simpler solution, and it would basically consists a mixture of existing powers, only smooshed into 1 set, (I'm thinking 2 from each element, with the last being a special build-up or toggle power that provides some special effect or bonus for each element).
2. A set that allows the character to freely switch between each of the 4 elements - each, individually, would be weaker than their single-element counterpart, which would be counterbalanced by some interaction between differing elements. For instance, maybe attacking an enemy with a water power, then electricity, would deal added damage, or fire then cold would cause some sort of thermal shock effect. It'd be much more complex to code, as you'd basically have to code like 16 versions of every power, (1 per element for no special interaction, then 1 for each of the 3 different elements).