21 not so exciting.
Looking forward to 22 which will have WebRTC, and new faster OdinMonkey JavaScript engine and CSS3 flexbox.
But I would really like to see it ported from Gtk 2 to Gtk 3 port, and support for <datalist> & <summary> elements, along with CSS 3 filters such as hue-rotation, grayscale, sepia, darken, brighten, etc.
Also better support for HTML5 form input types such as time, date, datetime, color, number, etc.