I am by no means an Automator or Applescript expert, but the Applescript below can be used with Automator to create reminders for each selected Mail message. Create an Automator workflow whose first item is Get Selected Mail Messages and whose second is Run Applescript with the script below.

The reminders are set to go into the “Follow-Ups” list with a reminder date of 9am the next day. If you want different values, it should be fairly obvious what to change in the script.

on run {input, parameters}
	set the_reminders to {}
	
	repeat with the_message in input
		tell application "Mail"
			-- If the subject starts with any number of "Re:" or "Fwd:", remove them
			set the_subject to do shell script "echo '" & the_message's subject & "' | sed -E -e 's/^((Re|Fwd): )*//g'
			set the_name to "Re: " & the_subject
			set the_body to "message://%3c" & the_message's message id & "%3e"
		end tell
		
		tell application "Reminders"
			tell list "Follow-Ups"
				-- Set the date to 9am tomorrow
				set reminder_date to (current date) + 1 * days
				set time of reminder_date to 60 * 60 * 9
				
				set the_reminder to (make new reminder with properties {name:the_name, body:the_body, remind me date:reminder_date})
				set end of the_reminders to the_reminder
			end tell
		end tell
	end repeat
	return the_reminders
end run

The script returns a list of the reminders that were created, which could then be passed to your next Automator action.

The work flow doesn't work, as it appears.

A '"' is needed a the end of the line starting
set the_subject to do shell script ...

The statements
tell list "Follow-ups"
end tell
are not needed and generate errors.

With these two changes, I got it to work! Thanks for the idea!

on run {input, parameters}
	set the_reminders to {}
	
	repeat with the_message in input
		tell application "Mail"
			-- If the subject starts with any number of "Re:" or "Fwd:", remove them
			set the_subject to do shell script "echo '" & the_message's subject & "' | sed -E -e 's/^((Re|Fwd): )*//g'"
			set the_name to "Re: " & the_subject
			set the_body to "message://%3c" & the_message's message id & "%3e"
		end tell
		
		tell application "Reminders"
			tell list "Follow-Ups"
				-- Set the date to 9am tomorrow
				set reminder_date to (current date) + 1 * days
				set time of reminder_date to 60 * 60 * 9
				
				set the_reminder to (make new reminder with properties {name:the_name, body:the_body, remind me date:reminder_date})
				set end of the_reminders to the_reminder
			end tell
		end tell
	end repeat
	return the_reminders
end run

> just open Reminders

If you're prepared to give Reminders a position in your Dock, you don't even need to open it first. Just drag the message over the Reminders icon in the Dock and it will

(1) open automatically and
(2) create the new reminder for you.

I think I might just leave Reminders in the Dock... :-)

This hint is helpful if you really want your email reminders to be time based. I know that was the reason for the hint but I rarely have the need to be reminded to follow up at a specific time.

In general, I find it faster & easier to use flags. I customized a "Follow Up" flag and assign that with a single click to any email (received or outgoing). Then I have a smart mailbox to show me all of them, which I can monitor at regular intervals (like daily) rather than being interrupted with a reminder when I'm doing something else.

I also like having all my messages I need to follow up to marked right in front of me in Mail.