Sure, a karma threshold works great for stack overflow (though it has lots of other details to encourage the OP to get it right). But I don’t think eliminating the ability to pick the subadmins saves you much programming effort. (It might be worth it to save administration effort.) Also, you need a change log to detect abuse of tags. SO does this by putting tag edits on the main change log.
Sure, a karma threshold works great for stack overflow (though it has lots of other details to encourage the OP to get it right). But I don’t think eliminating the ability to pick the subadmins saves you much programming effort. (It might be worth it to save administration effort.) Also, you need a change log to detect abuse of tags. SO does this by putting tag edits on the main change log.