Having the ability to change the UI/UX to make it look like twitters/facebooks one. Right now the current UI is not bad at all BUT I believe that the Wix team could use a little inspiration from the big competitors in the social media market.
As you can see, Twitter has many more features and a much better UI. Along with a profile page and the users feed many other things hide. An about section, a responsive and much faster mobile version and other huge improvements.