Here's my suggestion:
By default, without any mount options, use the current "legacy"
behavior. The devpts filesystem would point to a "global" instance on
the whole box, controlled by the traditional /dev/ptmx device node.
There would *NOT* be a /dev/pts/ptmx node.
If the devpts filesystem is mounted with a special option ("permount"?
"noglobal"?), then it will create a new devpts instance associated
with the filesystem. A devpts mounted that way *WILL* have a magic
/dev/pts/ptmx node.
If the kernel is built with CONFIG_DEVPTS_FORCE_PERMOUNT then the
traditional /dev/ptmx device node will be neutered (IE: always return
-ENODEV) and the "permount" option will be forced for all devpts
mounts. This will also remove the static global devpts instance.