Next year, on Commonwealth Avenue near Boston University, Elan will open the Academy of Hair and Skin by Elan Sassoon, slated to be the largest cosmetology school in the world, the first of four across the country. This month, he launches two high-end salons called Mizu, one here, at the posh Mandarin Oriental hotel, another in New York on Park Avenue. He's partnered on a line of spas called Green Tangerine, and rolls out his own product line next year.