It's tough to decouple the pre-existing condition part with the mandate part because then you're back to people waiting until they're to get insurance.
I'd see if rolling back the mandate to require catastrophic coverage only might work better.
You could also make the ban on rejections due to pre-existing conditions apply only if the person had some form of health insurance before.
For example, Aetna could reject coverage of Mr Smiths cancer because he was uninsured when he found out he had it, but not cases where Mr Smith found out he had it under when he was under another medical insurance coverage. So he gets the insurance, but Mr Smith has to get Medicaid or some form of government coverage instead in the first case, and in the second Aetna splits the costs with the prior medical insurance company. There are a million possible ways to do this.
Why cant we let insurance companies take on reasonable responsibilities and let the government pick up the slack when people make mistakes and fall through the cracks?