As much as I like watch them play Minecraft, I think it would be nice it they would occasionally play on other sandbox building games. Or play some Minecraft Mods that really change up the game. Such I would love for them to go back to Galaxy Craft, and this time go to other planets XD. It would be great if they were building on the moon attempting to build a ship to Mars.
Anyway, some other games that I think would be fun to watch them play would be...
Space Engineers Currently in Early Access. A game with zero gravity where they play as engineers in space. They have to build a base to survive. They can board other ships, mine asteroids, and have to watch out for meteors. A very good choice with lots of creative potential. www.spaceengineersgame.com/
Rust Currently it is in Alpha. A mixture of Wastleland, Dayz, and Minecraft, players must gather food and build shelters to survive. While I love this game, it is in VERY early Alpha. Key part of the game is the interaction with other players. However, sadly, until they add more creative elements to the game, I do not think there will be much potential for them to do a continuous Let'sPlay . playrust.com/
7 Days To Die I have not play this game. But from videos, it appears to be minecraft with more zombies, more realistic graphics, and physics! Just like in minecraft, players use blocks to construct objects. However, some physics and science is involved. Explosions can collapse buildings for example. 7daystodie.com/
I understand they are very busy with making minecraft videos and other projects, so I understand they would probably not get to these for a while, but I hope they will some time :-)