Creating Ages can be quite a challenge, yes.
First, you have to make your game moddable so you can link to your Age (I don't know about MOUL-based environments, but on Complete Chronicles, this is done using Drizzle and the Offline-KI, and is rather easy).
As for creating the Age itself, being familiar with Blender is already a huge asset for you
Then it depends what you want to do... At the moment interaction and puzzles are complex to create. Good visual quality is doable but comes with a few compromises (mostly, the light is "baked" into meshes and what you see into Blender might not match what you'll get in Plasma).
However keeping interaction and visuals simple doesn't mean your Age will be bad. People here made very cool Ages with almost no interaction and very limited visuals, but that span several kilometers with lots to explore. As long as the Age is interesting one way or another, people will like it.
On the topic of importing stuff... Yeah, the only version of Myst island we have is the one from Myst V. Like I said, we never managed to fully extract models from either realMyst games.
I'd recommend trying to import Myst yourself, it shouldn't be too hard:
- download and run the Korman installer
- download and extract my importer, and put it in <Blender's install folder>/2.xx/scripts/addons
- in the Blender Addons panels, enable "zero length zenith"
- manually create a TMP_Textures folder in Uru's DAT folder (this circumvents a small bug with the importer)
- and use File->Import->Uru import, choose a .age file.
Neither Cyan nor Ubisoft ever made a big fuss about people sharing their own assets, but it's still illegal so we try to keep it at a minimum as much as possible. Healthy caution, you know