WASHINGTON — Imagine if cigarettes were no longer addictive and smoking itself became almost obsolete; only a tiny segment of Americans still lit up. That’s the goal of an unprecedented antismoking plan being carefully fashioned by U.S. health officials. But the proposal from the Food and Drug Administration could have another unexpected effect: opening the door for companies to sell a new generation of alternative tobacco products, allowing the industry to survive — even thrive — for generations to come.