Now, AOL Everywhere
America Online, confounding skeptics, has emerged over the last two years as biggest, most powerful company on the Internet; an astounding 39 percent of Americans' time on line is spent using services company controls, 10 times share of its nearest competitor, Microsoft; despite success rate, Stephen M Case, AOL chairman, has plan to take company further that includes goal of roughly doubling number of subscribers over next five years, developing new services to lure users to stay on line longer and subsuming all other media by delivering its service on television screens, cellular telephones and other devices, in addition to computers; Case says that 'in the future, AOL is the next Microsoft'; photos; graphs (L)