One obvious missing modifier is the ability to animate them. There are a gazillion possibilities here, from the simple (slowly rotating or pulsing) to the complex (morphing one emoji into another). Some apps already do this when showing a reaction emoji for the first time by "popping" them into view, but I predict the next generation will allow the user to apply their own simple animations. The key will be to prevent them from being too annoying, like the dreaded <blink> tag of yore.