We don’t really have any current plans for extending ansible-test for more flexible collection testing at this time. The original reason that ansible-test became the standard for testing collections was largely because nothing else existed, so collections inherited the same rules as ansible-core.
Our goal going forward is basically to stop adding new testing functionality for collections to ansible-test, and move the ansible-test included with core back to only testing core. Which will require that we get another team on board with forking ansible-test to solely support collections, or to implement different tooling for the long term support of collections. In theory we have this agreement in place, but no timelines, work, discussions, or architecture have happened as of yet.