Also would like it so it is configurable in such a way that you CAN NOT select from your agency email address when sending in a subaccount for a client. i.e. users can ONLY pick from a list of verified email address and we can control that list

Not sure how it's going to work but It would be great that the "verified sender" emails will only be those that are non-free email accounts (i.e. NO @gmail, @outlook, @yahoo, etc.) since these free-emails affects email deliverability and hurts the MG account or domain reputation.

Also, are you able to find a way to restrict the sender and recipient email address to be the same? eg. I'm testing one email inside a workflow and I should NOT be able to use myemail@domain.com as the recipient when it is already the sender email.

Or (not sure how this will be possible) blocking a team member to get added to a Workflow if they use their own login email (as the recipient) when testing it.