This can't be done easily, and Gmail is particularly ill-suited to this task even for editing and resending one's own emails, let alone sending an email attachment to another Gmail user for them to edit and resend.
In Gmail editing pre-made messages is done using templates, but Gmail doesn't provide a way to share templates with other users.
If you send an email to someone they can save it as a template but that doesn't apply to EML attachments.
You might try the extension Gmail Email Templates by cloudHQ. I haven't used it myself but it was mentioned to me as a solution for sharing templates with team members, and the webstore page lists many of the features you need. Devil's in the details though, and I expect that both you and the recipient will need to install and understand it.
Approach Using Google Docs
My recommendation would be to change approaches. Instead of trying to share an email, share a Google Doc.
- Google Docs are easy to collaborate on and share with others
- Google Docs can easily become email drafts
- Your recipient will likely already have experience with Google Docs
Use Google Docs to draft emails
Source: Google Docs Editors Help
From your computer, you can use Google Docs to write an email draft and quickly preview it in Gmail.
Work with email drafts on Google Docs
- On your computer, open a Google Doc.
- At the top left, click
Insert > Building blocks > Email draft
- You can also type "@email" in the doc and press Enter
- To add recipients in the "To" field, type "@" and search your contacts, or type out email addresses.
- You can add a subject line, write text in the email body, and format your draft.
Preview and send your email draft in Gmail
- At the top left of the email draft, click Preview in Gmail Preview in Gmail.
- In the pop-up Gmail window, you can make additional changes to your email.
- When you are ready, click Send.
- Important: The email draft will be sent from the account you are logged into.
Additional Resource: Share files from Google Drive