I am currently playing on my xbox one, but when i eventually get a new phone, it would be nice to have all my progress transfer. I would think it would be fairly easy for atleast having cross save between pc and xbox, considering that microsoft has cross play with windows 10, but for phone and ps4, it would take a bit more effort, but hey, baby steps are necessary when adding new features to any game.
thinking more into it, you could make a 505 games app, where you could link all your profiles and games, that would be the biggest step towards cross save. A good example of a company that has a profile link system is callofduty.com; when you sign in, on your profile page, you can link twitter, facebook, youtube, twitch, xbox, playstation, steam, and nintendo. then you would just make a small step from there to where the save file saves to your profile instead of to the individual servers.