We could use the following variable indicators for A/B testing:
"From" name and sender address
64% of people say they open an email based on the "from" name over anything else (Chadwick Martin Bailey).
So you play against emails from friends / team members / clients / etc.
The subject line and preheader text
Length of subject line: Short, but powerful subject usually works best
Time and day the message is sent
Images: place / with- / or even without images - just plain text
Email message: length / tone / personalization
CTA: color / copy / placement / shape