I think it would be a big improvement for that module, if it would be able to ajaxify not only blocks but fields. To give an use case example:
I have a site with many views blocks on each page. All this blocks are the same for each user, with the exception of some fields: a rate widget, and some flags etc.
So, if I want to make the page cacheable, I have to load all that blocks via Ajax (or ESI), but wouldn't it be better to load only those fields that way.
I found a module that is capable to load fields via Ajax: https://drupal.org/project/peekaboo
Maybe it would be possible to use it's code basis to integrate this function in authcache?
Would do you think about that?