Ask HN: How to determine first killer app for new platform?

Suppose you are starting a new platform. It could be anything: a gaming platform, a hosting provider, a communications tool.

How do you go about identifying the first must-have application to build for and offer to users to entice additional usage?

If you think I'm asking the wrong question (i.e. this is too reductive), is there a better question I should be asking instead?