Twitter is developing a new video ad unit to support its soon-to-launch native video tool, according to marketing executives briefed on the initiative. While details are still being hammered out, Twitter is apparently leaning toward a “pay-for-play” cost structure. It would include a six-second preview video that automatically plays in user feeds and the option to click to view the entire video. Most likely, ads will only appear if and when users choose to view the full video, sources say.