This may end up being too clunky to actually implement, but I would LOVE to be able to comment on friend's community updates! Even with limited characters, being able to be like "I love this book!" or "This is on my TBR!" would be really fun. That, or custom/expanded reactions beyond just a simple like/heart.

I would also love to be able to comment on people's reviews—maybe a setting where people could say "anyone can reply/only friends can reply/no one can reply" could limit any potential fighting.