If the SCOTUS removes the insurance mandate, they are very likely to eliminate the Guaranteed Issue.
Guaranteed Issue means people can't decide to refuse to cover you for a pre-existing condition - or (almost as bad) - to cover you EXCEPT for the pre-existing condition.
Currently, in the US, all health insurance sold in the small group market (usually employers of 2-50 employees) is already guaranteed issue.
I suggest a simple rule:
Give Guaranteed Issue to anyone that has never been without insurance for a period of 2 months or less, starting at the time this rule goes into effect.
That in effect means you can't deny coverage for someone or even for a particular health issue as long as they have continuously held insurance from the law passage. I threw in a grace period of 1 month to make sure that simple screw ups don't kill someone's insurance.
It also means that every single American will have Guaranteed issue for a period of 2 months.
Yes, it will put a strain on insurance corporations for a short time as they are forced to cover sick people, but it will also give a huge intake of healthy people getting insurance right away to make sure that they will always be eligible for coverage.