Yasser Latif Hamdani at PTH has pointed out a terrible omission in the 18th Amendment Bill [PDF]. If you look on page 93 of the document, you can see that Article 91 is being amended back to resemble its pre-1985 status.
(2) The President shall in his discretion appoint from amongst the members of the National Assembly a Prime Minister who, in his opinion, is most likely to command the confidence of the majority of the members of the National Assembly.
The new version of Article 91, proposed in the 18th Amendment Bill contains Article 91(3) which is:
(3) After the election of the Speaker and the Deputy Speaker, the National Assembly shall, to the exclusion of any other business, proceed to elect without debate one of its Muslim members to be Prime Minister
So as you can see, the only good thing Zia ever changed about the constitution is being changed back and Pakistan will once again become a country in which non-Muslims are barred from the office of both the President and the Prime Minister. Either this is an error of omission, or an act of cowardice. In either case, it’s a huge loss for minority rights in Pakistan.