Some easy fixes, from someone who's written a book on the subject:
Get rid of the employer mandate. People can only afford health care if they're working full time.
Allow real competition in the ACA exchanges, not just as to copays and deductibles for gold-plated coverage, but also options for more limited coverage including catastrophic-only coverage.
Allow insurers more latitude to price policies based on age (the current 3 to 1 ratio forces the young to subsidize the old, and the cost of coverage causes many young people, quite rationally, to decline coverage).
Encourage simple employer health reimbursement accounts to allow employees to purchase individual health coverage in the ACA exchanges. That's the single most effective means for getting younger lives into the individual insurance marketplace and curbing the current premium death spiral. It also lets employers get out of the health insurance business and get back to making widgets.
DON'T mandate, as many conservatives want, competition across state lines. The states should retain their traditional sovereignty to regulate the insurance marketplace. But at the same time, free up the states to allow less costly, less comprehensive options in their ACA exchanges.
And, of course, the no-brainer - medical malpractice/tort reform.