Merge process / collaboration / pull request

I am not aware of any plans to add exactly what you are requesting, however in RTC 6.0.1 you could leverage the newly added Code Review feature to achieve most of what you want. The workflow would be as follows
1. Create a work item called "Merge Feature X to Release Branch Y" (i.e. to make it clear what people would be discussing/reviewing)
2. In RTC, change sets (in general) get delivered from a repository workspace to a stream, thus one person would have to bundle up the change sets they want to deliver and attach them to the work item. This may involve creating merge change sets if there are conflicts in the feature stream X vs the branch stream Y.
3. Enable the Code Review feature on the Project Area that owns the work item created in step 1.
4. In the Work Item (in the Web UI), on the Approvals tab, and under the Code Review section, click "Select Stream..." and choose the Release Branch Stream Y. This lets users know the intended deliver destination, and resolves all file/folder paths relative to that Release Branch Stream Y.
5. Now users can click on "Open Code Review" to review the proposed merge, and add "Issues" and comments on particular parts of the code. Or create "general" issues, which are a place to discuss things that do not pertain to particular lines of code.