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How about: "the aggressor in court must not only prove that the patent is valid, but also that it sells a product that uses this patent."
So if company A has a patent on a working product that it sells, and gets bought by company B, then we have three scenario's:
1) B bought A's patent and production line. Patent remains valid when going to court.
2) B only bought A's patent and has its own product that uses this patent. Patent remains valid when going to court.
3) B only bought A's patent but does not produce a product. Patent is no longer productised and hence becomes invalid when going to court.
So there can be no patent trolls. Hoarding and spreading FUD/blackmailing a la Microsoft would still work but the odds of winning a court case are much lower, so the blackmailing wouldn't be as efficient.