What a hiring committee for new faculty is looking for is evidence that the applicant will be able to continue a strong research trajectory over the course of (at least the) next 6 years, and become an established leader in his or her chosen field.
To make a convincing case for this, you need to include evidence both that:
a) you have succeeded so far in doing this: that is, you have a track record that if it were extrapolated linearly over the next 6 years would result in tenure. So you should spend a good deal of your statement explaining what you have done, and why it is important/influential.
b) You can plausibly continue your trajectory. To do this, you should express a cohesive
research agenda that ties in with your past work, and plausibly leads to a large number of meaty, interesting, unsolved questions. The more closely related this is to things you have already worked on, the more plausible it will be that you can carry it out. But it should also be different enough that it is new and exciting, and not merely a rehashing of your thesis work.
As in all statements, what you write about should form a cohesive story. If you work in several disparate areas, or would like to branch out to new areas, try and find something that ties them together, or some reason why your experience in both areas gives you an edge.