Anyone looking at this thread- I don't see any option to create a new repo for Assets only implementation but what I got recommendations from my peers is to use the full sites archetype and comment out redundant modules (e.g. dispatcher, ui.frontend, ui.tests etc). A possible reason why we don't have dedicated option is due to the fact that assets implementations can be augmented by certain add-on features like brand-portal, asset share commons etc. which follow sites like implementation. So these module might seem redundant but futuristic to support site features.