Thank you for reaching out to the Community with your feedback on our A/B subject line testing tool. I did want to first mention this tool is no longer in beta and is currently available on every 3GE template.
I also can definitely understand how helpful it would be to test other variables. I apologize, this is not a feature we currently provide. This is however a great feature request we have tracked in your account and submitted to our engineers! In the meantime here are some of our articles that goes over A/B testing in our system:
What is A/B Testing? Why It Matters & How to Get Started
Stage an Epic Battle with A/B Testing
Manual Subject Line A/B Testing and List Size
I hope these articles help!
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