@Gargron Yeah... Do you have a plan for the day someone has an interesting AS extension, maybe allowing some feature between folks using some popular client (at first, then other clients as they decide to adopt it)? Let's say, polls or events or items-for-sale or end-to-end encryption. I'd hope all servers would pass such stuff through cleanly, if they don't have something to do with it.
@sandro pleroma uses a document storage schema (jsonb within postgres) while mastodon uses a normalized relational database, so pleroma has an advantage in keeping objects verbatim even if they're not understood, at the cost of higher storage requirements.