I know this is an old limitation and we've worked around it by creating simply a plain text field holding the parent activity Id together with custom components (visualforce or lightning, both work) to render something similar to the standard related lists with the "New" button.
Plain and simple, the requirement is to have an UI showing related Tasks on Task and Events looking like the standard.
Now I found this Idea marked as delivered: https://success.salesforce.com/ideaView?id=08730000000BrPdAAK - but I can't find any in-depth documentation on the delivered scope or possible limitations on what has been delivered in Winter'16 exactly.
But reviewing other posts and what is possible in the Setup, my current conclusion is, that we still can't:
- create a lookup on task to point to another task. Use cases are sub-tasks for example.
- create a lookup on task to point to an event. Use cases would be tasks which are to be done in the context of the event (could be follow-up stuff on the event or prep-tasks or stuff to do on the event itself)
I find all these use cases very common and I wonder if we still need go the custom-component-way for that or what the most-standard-solution is at the moment.
Here is a screenshot of what Lookup-types I can create on Task. I'm missing entries for Activity and/or Task and/or Event.
I would like to verify, if this or something similar is possible or not. Workarounds welcome.