There are so many things you could do but my number one piece of advice would be to look at what your data's telling you - for example if certain content areas are getting higher clicks then others, look at what they have in common: is it a style of writing, use of image, colour of your CTA's, etc.
The real key to email marketing is to personalise using segmentation - i.e. make the recipient feel like you're emailing to a segment of 1. I don't know what product/service you're offering but look at whether gender, age or location makes a difference, then start looking at interest areas and build "interest-led segments' so if people have shown an interest in a particular product/service, keep emailing them about that but drop the other stuff....reintroduce it at a later date but keep focused on what you know they're really interested in.
My number two piece of advice will be to test, test and test again......