We aren't really targeting installation via composer. It may be doable with a custom type, but it's not something we're currently investigating. Generally speaking, composer isn't likely to be something available to a majority of our customers.Any chance you've managed to make addons self-contained and include-able via composer rather than needing to manually import an xml file after deploying files to multiple locations? Would be awesome to be able to host addons on packagist.
Failing that - I would at least like to see all assets for an addon deployed to a single folder so that we can use auto-deployment tools, and if there's still an xml definition file, a command line interface to deploy them so this can be automated as well.
We have isolated add-ons as far as possible, but that certainly also depends on the add-on. Add-ons aren't necessarily just PHP code; they may contain various external assets (JS, images/styling related elements) which need to be in a web accessible directory.
Add-ons are packaged quite differently and there are already some helpful tools for working with them.