I may bring an additional more top-down design perspective into consideration and to ideate on, but please let me know if that is better addressed in a separate topic.
What if we “Liberate people from clients, apps and servers”? 
If the “fediverse facilitates online communication and social interaction between people”, then the ActivityPub protocol specifies how to implement an abstraction on top of servers and clients in order to provide “a distributed network of addressible actors for the exchange of social activities”. This abstraction no longer relies on the notion of servers and clients, though for certain social networking use cases you may opt to re-introduce them (.. but then they live as concepts in this higher level of abstraction).
Though conceptually the ActivityPub spec didn’t prescribe this, the dominant abstraction for “joining the fediverse” in the present day involves “select an app, select a server that hosts your app of choice, and use a client that supports that app, to access it”. In other words, currently fediverse offers a predominantly app-centric view of the world. But this need not be the only view that is supported, and I advocate a shift towards having a fediverse of mixed apps & services, and elaborate a service-oriented view e.g. as a compliant AP protocol extension.
In the service-oriented view - and in a paradigm shift towards app-free computing - there wouldn’t necessarily be “Instances” as we have today, but parties that are positioned as fediverse access providers. You can compare this to the telecom sector, providing the plug in your house to deliver TV, phone and internet services at your doorstep.
When relating that to a good software with ActivityPub C2S support, I can envision that people after gaining access and becoming fedizens, can easily discover, obtain and wire services together in interesting ways that support their social networking use cases and needs. And dynamic UI/UX that supports that, based on service configuration and protocol (control) data exchange.
Update: See related toot thread.. https://social.coop/@smallcircles/114873942958303211