Molt had some pretty nice recommendations, but rather than suggesting games that play the most comparably to Unturned, I’d like to just suggest a game that I find to be as fun (even if it varies quite a bit in comparison), while still somewhat in a similar genre or feel to Unturned.
Cataclysm: Dark Days Ahead
Price: Free (standalone, download from website)
Singleplayer, third-person, ASCII/Tile based, open world sci-fi post-apoc survival, permadeath w/persisting worlds
Development: Released Jan 2013, latest update Mar 2019. New experimental builds come out every day.
Summary:
C:DDA is a shining example of a game with a seemingly simple interface but incredible complexity. The game itself is ASCII (though it also has an identical official tileset graphics alternative) and despite being graphically dated for obvious reasons, has depth that is almost unparalleled in the survival genre (though it also has a sharp learning curve to match). You play in a lore-rich post-apocalyptic world with a wide plethora of starting scenarios and world customization options, where zombies are neither the most dangerous nor the most outlandish entities you will see. The task of survival is a genuine struggle, and the scale and scope of the game world is a marvel, to the point that I’ve used C:DDA as a direct inspiration several times for my UII suggestions. You are free to play as you want, and while the world has a lot to discover, there is no obligation to follow the storyline. It’s 100% free, and frankly, 100% worthy of a recommendation.
Other honorable mentions: RimWorld, NEO Scavenger, Project Zomboid.
None of these are first person, but all of them have significant gameplay depth that is not apparent at first glance. (You might be starting to see a recurring theme in my game choices.) All three are paid, but well worth their price, especially RimWorld which has endless replayability and a modding community that is several times larger and higher quality than that of even Unturned’s. All three also deviate a lot in their own unique way from Unturned, but they all scratch the sandbox-survival itch for me.