Hello nicmar! I too find repurposing the exact same rig for different projects a bit cumbersome.
When I started to use Spine, we had a character done, and two more to create. We were two animators. I redid the new character while the other animator repurposed the previous rig. Repurposing took almost the same time (slightly more) than remaking the rig.
As long as things are simple it's not a big deal, but as soon as you start adding more advanced features or need custom meshes, it takes as long as just making it from scratch. We already feature several bipeds examples, and it's easy to see how they follow a similar structure but they're also all different.
In this regard, we do offer two asset packs: Spine: Asset Packs
And you can also find assets to repurpose on popular game assets marketplaces such as itch.io: https://itch.io/game-assets/tag-spine
Here's also a free downloadable dragon rig along with the stream in which it was created: Spine: Twitch: video YS90.3RFW9A
There's also the different views, art styles to considerate that can change a rig greatly.
You could also create a basic rig and then manipulate it to readapt it to various creatures with skin constraints:
Skins - Spine User Guide: Skin constraints