A big one is “How does an instance change their underlying implementation?”. Like how could a lemmy instance decide to migrate to become a Mastodon instance?
Currently that’s just not possible, but it seems important for the long term survival of an instance. It seems naive to think that an instance will stay the same implementation forever. But ActivityPub basically makes this impossible.
No it doesn’t? There is only one Lemmy implementation. There are some similar alternatives like PieFed and Mbin but those are separate implementations and are not in any way related to Lemmy, aside from using the same ActivityPub extensions.
There is really no such thing as a “platform type” - it’s all ActvityPub under the hood.