Lions are a keystone species, no?

I quite often have this fantasy where somehow the population of the world is eradicated by a disease that I'm immune to. And so, I immediately go and collect apex predators from zoos here in the UK and release them into areas with deer and other pray, releasing all cows, etc. And I open up all fields and fenced land around, etc. And then just allow natural ecostsytems to redevelop.