Good scenario: The CCP accepts a role as the biggest but not the only party in Chinese government. Zhao Ziyang stays on as General Secretary, Deng Xiaoping uses his political influence to keep hardliner factions from interfering with democratization, and the economy grows as IOTL but perhaps a bit slower due to less forceful centralized investment policies and inflated data. North Korea collapses a few years later with the death of Kim Il Sung. Negotiations with Taiwan to rejoin China begin and might look similar to the HK handover, with Taiwan being allowed to keep its political institutions and autonomy.
Neutral scenario: Surging mass movements all across China shock the CCP so much that it disintegrates without a chance to pursue rational political reform. Instead, the popular movement leads to ad-hoc democratic reform that is also terribly flawed and allows the rise of many Russian-style oligarchs who appropriate the state enterprises and turn China into a stagnant country almost wholly dependent on cheap labor for its economic prosperity. Territorial revanchism and and nationalism replace Marxism-Leninism-Maoism as state-approved ideology as a way to vent out the emotions of the people. There might be war with North Korea over something as stupid as insults between their governments (or as serious as ATL China simply not liking their nuclear program), or similarly a retake of the Sino-Vietnamese War, this time over the SCS. But if ATL China incurs western sanctions like Russia did IOTL, it'll be in much worse shape and may experience regime collapse when their labor pool is out of work. If Taiwan joined the mainland it'll probably try to stay as autonomous as possible and I imagine that TTL's China would be somewhat forced to permit it.
Bad scenario: The protests grow out of control as the Party leadership hesitates in the critical weeks of early summer. When the hardliner faction finally takes action, the civil movement is big enough that part of the PLA revolts or refuses to follow orders. The unpopular CCP loses control over many regional branches which defect along with elements of the PLA to the demonstrators-turned-rebels. Somewhat limited civil war with clear disadvantage for CCP. The rebels achieve victory sometime in the 1990s, but the resulting regime may not be democratic or clean. It is just as likely to be a South Korean-style junta, ruling over a wartorn and impoverished nation with the only selling point being that they aren't communist. More troubling would be the implications of a rump nuclear-armed PRC surviving in the Northeast. With its oil it would make a hand-in-glove ally for North Korea.
Chinese Apocalypse: Split off from above. Not enough of the PLA defects to the rebel cause and the two sides are more evenly matched. Warfare escalates to liberal use of heavy weapons and bombardments. Fearing that nuclear weapons have made their way into the rebel arsenal, the CCP decides to launch a first strike against the rebel formations and industrial/logistical centers. The rebels are prepared however and nuke Party-controlled areas as well. The death toll reaches at least 100+ million and every single country along the borders is affected as the war erodes Chinese political organization. Tens of millions of refugees overwhelm Southeast Asian countries. China is balkanized along say linguistic lines with the Mandarin-speaking part something akin to Serbia in the former Yugoslavia.