I really liked Rain World, but I'm also really bad at it, and so I ultimately moved on to other things without making very much progress. But now I have a reason to go back—several in fact. The 1.51 update, which was announced in June, is now in open beta on Steam, bringing with it new difficulty levels, multiplayer modes, and "new secrets, strangeness, and hopefully a generally smoother Rain World experience."
The new difficulties actually come in the form of two new playable characters. The Monk, "weak of body but strong of spirit," will round off some of the game's sharper edges: "In tune with the mysteries of the world and empathetic to its creatures, your journey will be a significantly more peaceful one," developer Videocult said.
For those who for some reason think that Rain World wasn't hard enough out of the box, there is the Hunter. "Strong and quick, with a fierce metabolism requiring a steady diet of meat. But the stomach won’t be your only concern, as the path of the hunter is one of extreme peril," the studio warned. Playing as the Hunter will also introduce a number of strange new creatures to eat—or be eaten by.
The update will also introduce a four-player competitive mode, in which players earn points by catching bats and spearing creatures (including each other), and a sandbox mode, also for up to four players, that enables creatures and items to be placed and played with so players can experiment and learn more about how they behave and how to deal with them.
To access the Rain World 1.51 update, right-click the game listing in your Steam library, hit Properties from the menu, and then select the Betas tab. Enter MANYSLUGCATS into the beta key field (I don't know if it actually needs to be in all-caps but that's what they said, so that's what I did) and then check the code. That will make "beta - new release!" available in the dropdown menu—select that, download the 280MB update, and you're all set.
Note that you won't be able to switch difficulties in an existing game: If you want to try the Monk or Hunter, you'll need to start a new game. Fortunately, Rain World has three separate save slots, so you can horse around with all three without having to give up on my progress made.