Merge fields for Campaign/Email Meta Data

We would like to see additional merge fields available.

For instance, it's pretty common to pass the %CAMPAIGNID% of an email message with the clickable link to track performance.

Same goes for the %SUBJECTLINE%. Some of this information is already present when adding "Add tracking to the links in this email": the utm_content param receives the subject line of the email, the utm_term receives the name of the group of contacts, utm_campaign receives the campaign name when inside a campaign.

The ability to add a ?email_id=[MessageID] is essential to properly track the results of emails and cross-reference that information across multiple platforms.