Three parties are involved, the withdrawal agreement between China and UK, and the special arrangement between US and China. Most likely all the complicated issues must be settled in the International Court of Justice and it will take a long time to settle as it affects all the laws of all countries.
Stabilising the RMB$ as it will be the most affected once the peg to US$ ends. Outflows from Hong Kong is expected but can be managed by capital restrictions and better interest rates.
Enhance the Stock Exchanges of Hong Kong, ShenZhen and Shanghai by offering companies better terms to raise money in China instead of Wall Street.
Move quickly to Digital Currency to prevent inflation and stabilise the economy and create better trading opportunities.
Accord special status of Dual citizenship for those who wants to return to UK or go Taiwan but do not allow them to liquidate their assets in Hong Kong.
Real estate prices are expected to take a hit in Hong Kong, as long as proper capitals controls are in place, the effects should not be more than 30% to ensure continued investments by keeping mortgage interests low.
If you can provide stability within 3 months, the reverse will happen as investors will jump in when they see there is no effects of the US move but instead create more opportunities to make money.
Hong Kong still remains the gateway to China and anyone who wants to do business in China still needs Hong Kong.