Roblox is known for its blocky, relatively unsophisticated character models and game graphics, but the style hasn’t dampened the company’s explosive growth. In his keynote, Roblox co-founder and CEO David Baszucki outlined plans to spruce up player avatars, introduce new in-game monetization streams and streamline the experience for developers dreaming up the user-generated content that turned the company into a massive success at the intersection of gaming and social networking. At its annual developer conference Thursday, Roblox painted a picture of what’s next for the fast-growing online multiplayer portal that’s like catnip for kids and teens.