Although there may not be a fourth domain of life, I am pretty excited about what the metagenomic studies that use next-generation DNA sequencing will fish out from the depths so to speak. There have been some crazy papers published that use this approach. The title of one: "Windshield splatter analysis with the Galaxy metagenomic pipeline." I imagine there is lots of interesting stuff living in the oceans that is about to be revealed using the high-throughput approach.

It is also interesting to think about what this approach will miss: if somewhere on Earth, perhaps near thermal vents deep in the ocean, however unlikely that may be, there exists a non-DNA-based life-form (which would easily be the most interesting result ever), metagenomic studies will miss it because by design they can only analyze DNA sequences. Similarly, if a probe gets sent to Europa, for example, DNA sequencing would be useless.