Hmm, what we have now is Contact Portal, i.e. a client could be your contact list and we can generate a contact portal link, sharing that link to the client will allow client to add some comment and discussion. However this section is not related to the task or a project. But yes thats a good idea to allow an independent client being able to add comment without being a co-worker. I will add this feature request to our Team.

I understand the need for public project. As it could allow outside input.
But then it becomes just a forum.
One feature I was thinking of on public project is enable a voting process on tasks to influence their priority or suggest new priority changes.

For for private project, technically, if they are adding task and notes they are co-workers.
What we can do is classify co-workers in defined types:
Employee, client, vendor, partner, freelancer, contributor.....

From there we could enable different options for each type.

They are both great ideas, thanks!

One more thing, can the viewing of private projects, can it be limited to a specific project or set of projects per user? Meaning a client can only see their projects and no other projects.

Yes, right now thats how it work.
If you share a project with a co-worker only that co-worker will have access to that specific project.