Hello,
What you are proposing is a kind of escrow service. Please see my comment here about this.
Could you please describe a use case? Why would this be better than an exchange?
The other kinds of blocking that you propose (country blocking, filtering of currencies, etc) can be implemented easily with the current Exchange. Please see this gist for an example. You can use this type of hook to implement any type of logic you like, to disallow a trade based on arbitrary conditions.
Thank you