Alternative titles: Users can submit an alternative title for a submission. If the alternative title gets more than X percent of submission's overall score it becomes the new title.
Merge requests: Users can submit a merge request by providing another submission ID or URL to merge with. Merge requests show on both posts. If the merge request's score becomes more than X percent of the lower-scoring submission's score, then the lower submission is merged into the higher-scoring one, maybe with small markers on merged-in comments to show that they were merged in, just in case the two submissions had a slightly different focus.
Mockup: http://min.us/lmLZve