I thought about building such a thing and have the right background. What do you think would be a good service to start with? I thought about k8s and then go on from there.
For networking not sure if you can/should integrate Hetzner's native networking and load balancers or should do some kind of overlay network like fly.io.
For managed databases and other services it would be great for you if first and third party "in your cloud" solutions like Aiven, Elastic Cloud, Snowflake etc would support hetzner but I've never seen it. Fly.io seems like a nice model of offering semi-managed databases.
With those things in place, and hetzner's few regions, some costumers might need Cloudflare in front of their services, but that might not be a bad thing. Is Hetzner in "Bandwidth Alliance"?