The ones I expect to occur are:
A housing crisis as landlords bail, government run housing turns into a crime ridden disaster and economic trainwreck. People revolt as Mamdani forces the use of public transit. If he pushes and gets massive new tax hikes to try and keep the budget under control, the major payers start to flee or find ways to avoid the taxes. The state balks at grandiose plans that cost billions and won't buy in. Crime skyrockets as the police are handcuffed by a criminal friendly government.
If Mamdani pushes his government run, and subsidized, grocery stores expect 'food deserts' to appear throughout NYC.
The whole thing will simply be disastrous and the harder Mamdani pushes Left, the worse things will get.