I'm not sure how far I am going to take this, but here it goes.
The first questions are - who would George HW Bush appoint as VP. There were several in Reagan's cabinet who would have been possible. Alexander Haig is an interesting choice.
Also, I think any timeline I would author would go under the premise that Bush was a spineless stooge who would have caved to the Democrat majority in Congress, so many of Reagan's successful policies may never have come to fruition.
Some items include:
The economic recovery that led to Reagan's landslide victory in 1984.
The Grenada Invasion
The Cold War
Going further into this time line it is worth exploring the election of 1984. Would Mondale still have been the nominee. If so he certainly would not have chosen Ferraro as a running mate.
Hopefully, I will provide more insight at a later time.