The diff of <parent_revision> vs. <new revision> should be added to RCStream. This will be useful for a wide range of wiki tools.
Diffs are CPU intensive, so they haven't historically been generated at the time that an edit is saved. However, tools are already sending requests to the MediaWiki API for every revision made in English Wikipedia using the RCStream. It would be very beneficial to be able to skip the additional request that causes the diff to be generated.
Given the computational complexity of generating diffs, it's expected that this will cause RCStream to lag by a few additional seconds. This would be acceptable for the applications that are currently hitting the API for diffs since they are currently delayed for the same reason. However, it might make sense to separate the Diff-RCStream from the Metadata-only-RCStream.
Save times are a potential concern here. It was also noted in conversation with Ori and Yuvi (drawing from memory) that RCStream is emitted as a post-save hook so as to not slow page save times. It seems that the diff could be generated and new RCStream events submitted via a similar post-save hook.