I still think a "Move to root" would be nice. I'm not sure what a left/right button would do in this case. How do you see up/down working in this case also? I normally see those in ordering situations vs a hierarchy situation.
To be honest, the scene tree needs an overhaul. It has to be structured so differently to give a nice flow. Maybe allow us to make folders, which are only for the editor and have no game purpose? It just gets so sloppy with big scenes. This nice thing is this is only editor functionality and nothing to do with the game.