PGP already allows for this via "owner trust". When you sign a key, also indicate the level of trust you have in newly certified key's owner to certify other keys.
It's of course up to you to decide who you can trust to certify other keys.
edit:
It's worth mentioning, "owner trust" is strictly a local attribute -- just because you fully trust John, and I fully trust you, my trust for John's certification of 3rd party keys remains unknown.
It's of course up to you to decide who you can trust to certify other keys.
edit: It's worth mentioning, "owner trust" is strictly a local attribute -- just because you fully trust John, and I fully trust you, my trust for John's certification of 3rd party keys remains unknown.