BIG RANT (MY POINT OF VIEW): I don't think the creature that's in the tree posses the power anymore, that's if it WAS alive in the first place since we clearly saw it disappear in the manga. Eren erased the power of the Titans and "the source of all living matter", and kept paradis safe for 20000 years before the cycle of war and basic nature took its turn on paradis like it does with everything else. Also, i think the "to you 20000 years later" said by Eren to this boy is either symbolic (aka not canon) or this boy sought to revive paradis once again (similar to Eren's purpose), basically having something about him that relates to Eren, so isyama wanted to make that connection. Not that Eren actually talked to him, it's more like there's this similarity between them or that this boy understood Eren, like Eren understood ymir. Except: 1) Unlike Ymir, Eren wasn't desperate for someone to understand him after his death, As he knew his friends did, the ending "name of love" seems to shed light upon Eren's feelings to his friends. And 2) The Paths are gone, which should be there is Eren was to communicate with the boy, Also the Hallucigenia is gone. And so is the power of the Titans. They're gone for good.