I was considering roleplaying videogames and common problems people have with them, mainly the length, repetition, and slow gameplay which turn-based RPGs tend to get picked on for. I wanted to think of an RPG design that would solve those issues.
I don't think this would be a game that could please everyone, of course, and I know that some of these things are big reasons some people love RPGs, but I still want to consider what a short and fast RPG would be like. So I went looking: GameFAQs rankings, roleplaying genre, sort by length, low to high.
The first item on that caught me by surprise, but in a "I really should've seen that coming" sort of way: Deltarune Chapter 1. It was listed as something like 5 hours long, but I had played it before and I know you could easily beat it in 30 minutes if you know where you're going and skip dialog. So yes, a short, fast, turn-based RPG, but Deltarune Chapter 1 is only one eighth of a game, so I looked further down the list... and Undertale was not far from it.
Undertale, clocking in at about 5-15 hours (I looked up some playthroughs, neutral run was 5, pacifist was 10, but GameFAQs listed it as 15) definitely has stuff we can learn from: we can see that a full-fledged story can absolutely fit in a 10-hour playtime, for one. Another thing I notice is that the battle system is very unlike other turn-based RPGs. Well, it looks a lot like one on the surface, having a small set of stats, items, equipment, and so on, but the star of the show is the bullet-dodging between turns; it makes it so that you're never really waiting in a battle (unless you're reading dialog) like many other turn-based RPGs and the action adds a whole new layer to the gameplay.
That's cool and definitely contributed to the game's wide appeal, but I sort of see that as a problem, or more accurately, something that doesn't help solve our problem: it doesn't make turn-based combat less slow, it just grafts another mechanic on to there. I'm not saying that's bad, mixing mechanics and systems makes for some of the coolest games out there, but if you play Undertale and your favorite part is the bullet-dodging, you're probably not going to like Final Fantasy or Persona as much since you have much less of an active role in combat in those.