Some sort of read-only toggle for a realm.
 

Again, I don't know how difficult this request is. But if the effort is reasonable:

Say I've set up a structure that I want to use only as a template. I don't want to accidentally add content to it.

Or say I build my base world, and that's my gold copy. I don't want to accidentally load that realm, and run an adventure for a half-hour before I realize my mistake.

Obviously, I would want the ability to edit and update these realms, but having the ability to toggle a read-only mode would be a nice way to help ensure that I edit it only when I want to edit it.

Sounds like a good idea but seems like you could rename it to something that would cause you to think twice ("The Never Realm?") as a stop gap

The realm selection screen has a crown icon for realms. Perhaps a different icon for templates or frameworks?

That's a fantastic way of differentiating things without it being overly obtrusive, I +1 this!

While I support this as an elegant workaround, I think the OP's request should not be too complicated to implement. And it can get a nice padlock icon on top :)

I agree - +1 to all of the above

Indeed, +1 to the above.