Will commission-free trading be available to Singapore clients?

Yes, sooner or later. It's only a matter of time. My guess is within the next 3 years.

Nonetheless, while the per-trade commission or fee may drop to zero, we should not expect totally free services from the brokers. The service provider has to make money somehow (from the client, from third-party, etc.) in order to make this new model sustainable in the long run.

The "new" form of charges could be like monthly fees based on average AUM, tiered pricing based on trading volume.

No, it won't.
Local brokers are charging higher fees and custodian fees so there is little competition for overseas brokers to offer free brokerage fees.
Furthermore, we are not able to open bank accounts with these foreign brokers due to the patriot act and thus they have fewer incentives to serve us.