Another one with a good demo during Steam Next Fest is Paris Transylvania, a pachinko roguelike deckbuilder. The demo has Native Linux support (although it only shows a Windows icon) and the full game will launch with Linux support.
I have a feeling I'm going to be terribly hooked on this one. I absolutely love Peglin and it does give off a similar feeling but comes off quite unique with the deck-building and relic system. So it's a bit more like a combination of Peglin and Slay the Spire in a way and that's great. Being able to play your cards in any order is also an interesting twist, allowing to to plan ahead a little more for some fun combinations.
Check out the trailer:

Direct Link
Game Highlights:
- Dynamic Deck Building - Create a unique deck by choosing cards wisely. Discover hundreds of cards to defeat wicked creatures and regain power.
- Pachinko-style Gameplay - Strategically hit pegs to power up your cards. Uncover game-changing relics throughout the world to enhance your deck.
- Ever-Changing Transylvania - Maps change with each journey. Choose paths, face different enemies, encounter random events, and confront various bosses.
Speaking over email one of the team mentioned they would "love feedback and as a small 2 person team have limited options of game testing on Linux" and that they're "very happy to have Linux users in our community, and solved the first detected bugs fast".