if the image for a :shortcode: changes locally does it also change in the federation or do those two just come out of alignment because that's confusing
if the image for a shortcode changes does it start federating differently so the same toot will have two completely different appearances on different servers depending on whether they received it before or after the change or
you could argue that Unicode emoji work the same way (ie appear differently on different platforms) but the difference is that those are backed by a standard and vendor agreement which clearly defines them and also can be historically researched and pointed to at a specific moment in time

OH, AND ANOTHER THING:

seeing emoji in my feed that I can't put into my own toots is really bad UX

the first thing new users will say is "oh, how can I insert X" and then the response will be "you can't" and then they will say "well, this is shitty software"

this is why i was advocating an opt-in, non-federating solution because it means (a) every emoji will be vetted by instance admins, and (b) every emoji a user sees is insertable, which to me seems like a common user expectation to have
@kibi A non-federating solution sounds unpleasant to me too, though. What about an attachment to the status (as if it were other media) for any nonstandard emoji? Maybe paired with some UI for copying them to your own stash of them. (And an instance-configurable set as well, of course.)
@kibi @lain posited that it might make it more interesting if servers have exclusive emojis that give that server flavour. Kinda congruent with Discord cc @bea