Client's should be able to add messages to a project, referencing a file or task. Additionally, admins should be able to respond to these messages. I know tickets is a way to do this, but it's many more clicks for both admins and clients to respond there and then check on the status on the project overview.
Client's should also be able to add descriptions to tasks they add, so that they don't need to add a message or open a new ticket. They should also be able to add a file to a task so that they don't have to reference it elsewhere.