My suggestion for HubSpot is to allow for us to easily toggle between A/B versions of an email when previewing. It gets confusing when you have to exit the preview screen, switch over to a different version, and then try to ensure that the differences between the two are correct. It's easy for things to get lost in the mix with all the moving around that's needed to compare versions. Also, I would like to see subject lines A and B shown together when reviewing before send.