Hi anonymous question asker. Bad news for the designers out there: not much.
Emails have the digital capability of a sausage. They are literally stuck in the past as vast swathes of the market are still stuck on old versions of Outlook which uses a version of Word to render rather than a browser. As such only basics really work easily.
Adding for example a gradient background means that any text over that area would need to be exported as an image rather than as live text, obscuring it from view on email clients which require you to turn on images to view and also makes this a design job each time for any changes to the email rather than simple html text changes
Background images don’t work. Extending things 100% width to section it out doesn’t work (as evidenced by your current emails which have horizontal scroll bars on Outlook)
Whilst you could argue that certain styles like a drop shadow could be applied to the foot of each section only to more capable email clients. With a responsive email design you are adding in a far greater margin for error with how the email handles in the hundreds of email clients available. Increasing QA and build times exponentially.
After having tried all these things previously I know from previous results that they have little effect to CTR, they generally just create headaches.
Animated gifs may add a bit of pizazz but again Outlook won’t render the animation showing only a static of the first frame so any essential messaging/CTA needs to be on the first frame, also Outlook will probably initially block images. Apart from this, support is actually pretty good for animated gifs and may be the clever way of adding in a differentiation that can be handled at design stage without adding to much time at build. Asos are using this technique quite a bit for their product emails but I can see this being more appropriate to their needs as it allows you to see different shots of products without increasing length. I haven’t any stats to show the effectiveness of this techniques yet. Here is a url about it though with words written by someone probably cleverer than me:
As always, for me it’s more about about a compelling subject line, offer copy and simple postcard style email with less distraction anyways to not waste the users precious time & push the users to the conversion goal with all of this verified via A/B testing
 As you may have guessed, this is a response to an email that came in at work. Having concluded writing it I obviously thought these opinions may be useful later on.