For starters: I look for the most obscure of the obscure. We're talking about English demos whose reviews are in the single digits. I've gotten better at finding the good stuff in a sea of crap, and I'll share some of my findings here. I will also mention the two games I've posted here a few days ago.
Action Adventure
This is a Westworld-esque game made by a Spanish speaker where a rich kid school brings in some waifubot helpers to help the students, but the waifubots go crazy and start attacking people. The demo continues up to the start of the revolt, and there's some decent worldbuilding and foreshadowing leading up to the big event.
The trailer doesn't really do the demo justice. The environments look basic, but the animations are actually pretty good. The voices ingame also have a lot more inflection than the crappy monotone voice in the trailer, and there's a basic dialog system with some of the NPCs you find to get some lore.
This is an exploration game. The Moon is going to crash into Earth in about 30-40 years, and no one can do anything about it. What's more, some of the dead are still attached to Earth and unwilling to leave, so the protagonist has to use his musical instrument to make the ghosts pass on.
A big theme of the game is about choosing to press on despite the impending destruction of Earth. Tides have ensured that the only walkable land left is a little island that used to be a mountain peak, and humanity is reduced to a tiny village by the coast. The game gives me some big Aniara vibes.
Visual Novels
This one is about a village that looks okay on the surface, but something feels off. It has a supernatural theme, and implies that some of the characters encountered aren't as decent as they seem to be. The mystery is neat, and the demo gives you a few hints about what's going on.
On the surface, this looks like a typical dating sim. But this one has a twist: Some girl got bumped off in the changing rooms, and your character got called in to find the perp as a favor for the principal. From what I understand, the goal is to solve the case via the dating sim mechanics; the goal is NOT to use the dating sim mechanics to push a girl's affection up to max.
For example, there's one scene where your character can choose to get distracted by another character, or look around the room. Looking around gives you a clue. There's another instance where you get an opportunity to inspect a character closer, and doing so gets you another useful clue. Clues are stored in a notebook you can review at any time, though I haven't seen anything that would let me use said clues in the demo.
Honorable Mentions
These demos did not participate in Next Fest, but I feel they're worthy of mention.
This is a spinoff of a previous game the dev made (The VII Enigma). It's set in the latter half of the 21st century. The player character is a specialist called in to investigate a specimen in a cave on a faraway island. However, bad things happen, and the guy ends up trapped in a collapsed cave. It gives me The Descent or Time Trap vibes.
This is another mystery visual novel, and is much closer to Ace Attorney than Rose Academy above. A member of an idol group goes missing, and then turns up dead. You get called in to figure things out. Later on in the demo, you get to use the evidence you collected to bolster your claims.
A bunch of college kids drive out to a secluded mansion owned by their professor, for extra credit. Eventually one of them goes berserk from the stress and starts attacking people. This visual novel is a remake, and now provides English.
The player character also has a cute notebook you can look at, which notes the various happenings of the day. It's a neat touch.