The first year of enrollment under the federal health care law was marred by the troubled start of HealthCare.gov, rampant confusion among consumers and a steep learning curve for insurers and government officials alike. But insurance executives and managers of the online marketplaces are already girding for the coming open enrollment period, saying they fear it could be even more difficult than the last. One challenge facing consumers will be wide swings in prices. Some insurers are seeking double-digit price increases, while others are hoping to snare more of the market by lowering premiums for the coming year.