Hello 
This is a really great idea 
I think it’s might would be good idea to merge the topic map with timeline somehow.
I made a rough mockup about the posters how looks like on timeline. This can suits with embedded replies design. This section would always on timeline and dynamically change during scroll or clicking the avatar to always show the avatars in the current area.
This way the topic map can reduce. The avatars can remove from there and move to timeline. (of course smaller size) 
but the active reply avatar can be bigger. Something like on MacOS Dock Magnification effect.
The main avatars on timeline is depends on likes or reaction and if there are replies between avatars then it can show the number of replies clicking on that expands the area with the avatars. Something like on maps.
