One of the most important template locations for which a standard hook is needed is post content; primarily, allowing for injection of code/content after the post content.
I would recommend any/all of the following template locations for consideration:
post_content (or post_entry or whatever)
Or, using the existing entry nomenclature, perhaps:
...with _before and _after variants for each.
p.s. I'll gladly do a pull request; I just wanted to get the concept blessed before doing so.