A good question.
Mugabe essentially focused all of Zimbabwe's government around him, which is why now its impossible for the MDC to get him out without widespread violence. In 1990, he's not quite complete merging ZAPU into ZANU (PF), which leads to the question of Joshua Nkomo trying to take over.
1990 also saw Edgar Tekere's Zimbabwe Unity Movement make a strong standing in the polls, which complicates the question further.
Mugabe's successor would have to keep the country under his grasp, which considering Robert's leadership style would be a real problem. Tekere's ZUM drew lots of support from ZANU (PF), and if Nkomo can regain the Ndebele support the two together could well topple ZANU (PF), assuming this all happens before the 1990 elections.