In some states I understand the Advantage Plans have already been canceled.I don't know what the future holds with regard to seniors' coverage, but if the Advantage Plans disappear, there's also Medicare Supplement Insurance.
Government sponsored, they're often called "MediGap" plans and cover up to the full 20% that Medicare doesn't.
Medigap Policy Search
***Medicare Supplement Insurance. Government sponsored***
Medicare Supplements are not govt sponsored - they cover the 20% Medicare does not cover and are paid by the Medicare recipient - usually about $150 - $200 month - to a private insurance company [like AARP (shudder)]