For postmortems the basis is having document templates that are linked to the incident. From there the ability to automatically add information to the individual postmortem documents with things like timeline and MTTA/MTTR. Add the ability to comment on, share, export, and collaborate on these documents.
The second piece of functionality is around action items. Almost all postmortems generate action items. I need a way to tie action items to a specific post mortem and integrate automatically with my project tracking software (Jira, Asana, Linear, whatever). Ideally there's some top level reporting functionality that shows me the status of post mortem action items.
A nice to have would be automatically scheduling the post mortem meeting with a calendar integration.