1989
Instead of the unpopular and unable polit buro puppet Egon Krenz, the more competent, moderate, and popular Hans Modrow becomes the new head of Eastern Germany (Modrow got the position on paper, but not in reality IOTL).
Modrow, being dropped by the SU, loosing support among the people, faced by rising emmigration, and so on, decides to put everything on a single card: He holds a popular referendum whether to stay independent or apply to become a state of Western Germany.
At the same time, he also does the much needed economic and political reforms - more free market, more political freedoms, more civil rights, democratic elections, and so on.
The referendum turns out pro independence by chance (not unrealistic - even among the protesters of the time, many didn't want a unity with Western Germany). Efforts by Western Germany to push unification, western political parties, and so on, derail.
Borders are opened, many people flee to the west, but most stay. The newly opened borders and liberalized laws even make some emmigrants return.
All those liberalisation efforts will not go too well with all the hardcore communist brickheads in the SED. Modrow manages to break up the party into a Socialist, a Social-Democrat, and a Social-Liberal Party. He becomes head of the Social-Liberals (probably mainly moderates who only joined the SED for career reasons), which surge in popularity, while the Socialists become an obscure splinter party.
Unlike OTL, the wages aren't increased artificially high, and buying of companies by western companies just to demolish competitors will rarely happen. Therefore (and for a few other reasons), Eastern Germany draws even more western and other investment than IOTL, while at the same time loosing less business to eastern and southern European countries.
Many highly qualified western German jobless people who aren't eligible for jobless benefits (too much property or other reasons) even work in Eastern Germany (despite much lower wages), thus softening the impact of all the emmigration from east to west, and adding lots of know how.
The Eastern German Republic becomes a success story, Hans Modrow even manages to get elected for a second term (his first with democratic legitimisation).