What about a Voldemort who comes back and just Avada Kedavras Harry on sight? This would finally do justice to Voldie's superior power and intelligence, and take only one paragraph.

I was indeed thinking of spells or artefacts we don't know about (or perhaps clever combinations/side-effects of magic we do know about). If Horcruxes are indeed images of the caster impressed upon the world then perhaps it might be possible to construct them in such a way that they become active unless repressed by the original? That would solve Voldies stuck-in-a-plaque-for-decades problem as well.