A plan that could
I see this "that could" part (with no complement) everywhere, as in:
The Little Engine that Could.
What does this phrase mean?
That could (or that can) by itself is an idiomatic usage that has an implied verb that is something like succeed. The Little Engine that Could keeps repeating the phrase "I think I can [make it over the mountain]", and the headline you mention means that this is "the plan that could [solve the problem]".