That's a nice idea, but it gets a bit tricky with the reality of landscaping. A lot of these cemeteries are way too densely packed to allow one tree per person. You'd need to group people up. If we're going to group people up, we might as well make one big marble mausoleum that contains the New Yorker Ash Heap where we can all mingle like rush hour 6 train commuters for all time. Designing a park around bodily remains will always be cumbersome and will also creep a certain number of potential visitors out. People get called out all the time for taking overly frivolous social media posts at cemeteries - there is a stigma for many activities in a burial ground. Remove the bodies and make them real parks. The Big Ol' Ash Heap can go somewhere cool like Randal's Island or the interior of the Statue of Liberty.