Right. I like to use WordPress as an example of a successful ecosystem that has worked out a good model for this. The WordPress core uses the minimum set of features that all websites likely need. Beyond that, there are plugins that people can add to their WordPress websites in order to get additional functionalities that they're interested in. WordPress core sounds very much like HAF, whereas additional projects like the ones you're building sound very much like HAF plugins (although they need not necessarily depend on HAF).
Now, the customizability part. Every website (or dapp in the case of Hive) has its own layout, its own design and workflow. To tackle this, there is the separation between the logic of a plugin and the presentation (frontend layout). In WordPress, this is solved by having themes - just by changing the theme, the presentation of a plugin is greatly changed, even though all the functionality is still there.
Furthermore, to additionally enable an extremely powerful level of customizability, there are hooks all over the code of both WordPress core and plugins. The hooks allow for changing any part of the behavior and appearance. So for example, if a dapp uses the chat functionality, it would be able to easily tweak any aspect of the appearance and functionality, and even add additional functionalities. This way, more than 95% of the work is already done and the dapp has to do only the remaining 5% or so for customizing it to their own needs.
This also helps with keeping things up to date - when there are updates to the plugins, the dapps can simply update to the latest version and in almost all cases everything will still keep working. This will also greatly incentivize dapps to contribute back to the plugins, with code and/or funding, so that they help the overall ecosystem.
All of this is already happening in practice with the WordPress ecosystem, I'm just saying we can take those lessons learned and apply them here.
Are there design documents or specs for the projects you are working on that one can contribute to?