Adventure In Aellion is an upcoming drop-in / drop-out co-op adventure game, which the developers claim 'evokes the spirit of Legend of Zelda'.
With a big open-world to explore full of secrets to find, dungeons to explore and more it can be played entirely in solo. However, it will also have cross-platform online play so that others can join you to solve puzzles and help with battles.
Visually it's using a retro low-poly approach which actually looks quite good thanks to the consistency, with the Adventure In Aellion trailer shown below making it seem pretty fun and at times a little amusing. I'm looking forward to trying out some of those puzzles:
They've released a free Beta recently, which is not available on Linux. However, I've spoken to them and they made it very clear the actual game will be when it goes live as it's still in testing. Their current plan is to enter Steam Early Access on July 22.
You can follow it on Steam.