@evan no joke.
I think that adding new properties holding only specific types of activities/objects is not a clean way of doing it.
What happens when I want to support EmojiReaction activities? Do I need to enhance all supported object types with an /emojireaction property?
I would see a FEP to obsolete /shares and /likes in favour of /reactions that can contain any number of activities (Announce, Like, Dislike, etc.)