Constituent ID merge field would be a dream as we do some surveys and voting forms that require this ID in order to access the form as you must be an alumni and this is one of the checks put in place to give access to the survey/voting forms. If we can't merge this field into a marketing email to a targeted alumni base, then it means we have to straddle between two mailing systems.

Also input Membership information as merge options so we can send renewal notices.

This was a huge reason we switched to RE (and they promised it could be done). Obviously the sales team was working with a version a decade in the past, as renewal notices aren't achievable with RE anymore.

Agreed. And there should be scope to add a Custom Field merge as well.