Opdivo from Bristol-Myers Squibb ($BMY) followed Merck's ($MRK) Keytruda onto the market, but the immuno-oncology drug quickly overtook its rival and has never looked back. It just today won recommendations in Europe for two more indications. But while it leads the race, analysts say there are still opportunities for Keytruda and drug candidates from others to close the gap in a market that is projected to reach $40 billion in annual sales.