Just as Yahoo gets one competitor off its back by quitting merger discussions with Microsoft, the Web portal may soon find itself going head-to-head with a new rival. Starting June 18, News Corp.’s social network MySpace is introducing design changes it hopes will make it look and feel a lot more like Yahoo. If all goes to plan, MySpace would go beyond being a site where people build personalized profile pages and hang out online with friends, and become more of a gateway to the Web, where users can read news headlines, listen to music, watch videos, and more easily communicate with pals.