I think if Sharif's are really serious to save their government. They have to take a couple of steps.
Starting from the top level government functionaries, they need to change the President. The President could be from interior Sindh, Baluchistan or KPK. Lots of candidates could qualify for this post as Mumtaz Bhutto (Sindh), Ghaus Ali Shah (Sindh), Pir Sabir Shah (KPK) etc.
They should appoint a Deputy Prime Minister as Zardar had made Pevaiz Ellahi Deputy PM. Pervaiz Ellahi should be given the same portfolio again. This will break any opposition in Punjab. PTI is the main opposition in Punjab, but they wouldn't go to any position under Nawaz Sharif. Punjab is the main strength for Sharif's. To make Punjab its main stronghold, the biggest opposition should be given a Deputy PM position.
MQM should be given a position in the center government e.g. Speaker National Assembly, Chairman, Senate etc. MQM won't accept these positions they would rather more interested in business portfolios e.g. Chairman Steal Mill, Chairman Shipping Corporation, so let's go with their choices.
People party should be made a part of the cabinet in the central government, so that they should look stand with the government. This should be reciprocal to their government in Sindh. If they want Sindh government, they should also be a part of a central government.
ANP should be given a share in the central government, and if they want give them the governorship of KPK. They could counter PTI in KPK.
Fazalur Rehman should be given more active role in the government, e.g. Deputy PM, governor KPK.
People party has given Gilgit-bultistan the status of the province, but I don't think they have any representation in the National Assembly and the Senate. Gilgit-Bultistan should be given representation and Sharif's should try to make PMLN active in Gilgit-Bultistan.
Punjab is a big province. Southern Punjab has a long term demand of separate province. Sharif should make this historic decision and make MLN a strong party in the Southern Punjab. This could increase strength of PMLN in the province and in the center.
MLN should immediately announce local body elections in the country.
This solution is based on the formula of power sharing. Unless and until PMLN don't focus itself on power sharing, they won't be able to stay in the power. Power sharing is the power practicing. If PMLN want to stay in the power, they have to share the power.
Royal style of governance Sharif's is practicing won't last long.