I've found that playing RPGs on the phone is nearly impossible.
Well, to be more precise, it's extremely easy to play them when I have the time. When I was stuck in my parents' house for holidays, I could play RPGs quite well - long stretches of nothing better to do. So this is not an interface issue, it's an issue of time. My normal play habits involve ~20-30 minutes stuck on a train. Too short and too loud to get immersed into an RPG.
Instead, I play casual puzzle games such as Threes.
I like Threes well enough. But I miss the depth of character and world that comes with RPGs.
On a computer or console, the RPG is structured with a specific kind of pacing. We've polished that pacing until it sparkles, but it's becoming clear our standard RPG pacing won't work in small bursts. At least, not for me.
Rather than creating "a casual RPG", I'm thinking about ways to deliver the same characterization, world weight, and personalization that RPGs offer. But in small bursts.
One way to do this is Animal Crossing. The casual play of Animal Crossing and every game like it is compatible with short play sessions, and as you play you will slowly come to know and have feelings for the various other citizens of the town.
The play of Animal-Crossing-style games is largely maintenance. That is, there are various timed resources and you wander around collecting them. In some of these kinds of games, the timers are realtime. If you want to play for a long time, you start to explore areas you usually ignore. In some games, the timers are in-game, and the game world can easily be advanced - for example, farming games. In this case, the play is broken up into chunks of your own choice.
I prefer the latter idea, because I know how long I have to play. If I have a lot of train ride left, I might decide to tackle two days. Or, I might decide to do the maintenance stuff and then spend some time wandering around doing optional things. If I have less time, I'll naturally be able to focus on what I need to get done, and then end off naturally by going to sleep as my train arrives at my stop.
This player-selected duration is powerful, because not only does it allow me to pace myself based on the time of my travel, it also allows me to pace myself based on my current mood. If I'm impatient, I can push from day to day. If I'm feeling dreamy, I can wander the world without advancing time at all.
Fundamentally, maintenance-based gameplay is not the style of play I feel compelled by. In addition to not feeling as interesting as I'd like, maintenance gameplay is also contextually limited: it doesn't feature any significant travel or change, so the world can never be pushed into my head. It can make me enjoy characters, but the world will always feel tenuous because I'm stuck in a single spot where nothing really changes.
A game like Threes takes another approach: it's a skill-based puzzler. The puzzles are 2-5 minute affairs, and you just keep repeating them.
I believe the concept of a puzzler can be adapted to allow for character and world development. But it can't be pure. A pure puzzle (like Threes) can't really have characters and worlds hung behind it. The pure puzzler doesn't entangle easily: it has nothing to do with other characters or specific places. It's pure logic.
So there are impure puzzlers. "Fuzzy puzzlers".
For example, Phoenix Wright games feature extensive character development and are a lot of fun. They are not ideal for commutes because their pacing is too heavily structured: I can't deviate from the pacing depending on my mood or my allotted time, and the viable break points are too widely separated. Also, the world development is pretty scarce, largely because it's just not a focus.
The core idea is good. A puzzler that involves interacting with weirdos as its core mechanic. Phoenix Wright has terrible gameplay, all considered: there's only one path forward and it's basically a test of how well you can read the developers' minds. But the terrible gameplay is simply a gating mechanism to pace the content. It makes sense to have a strict linear progression when you exist solely to show off your fun characters.
That style of gameplay doesn't allow for customization, though, and that's a core piece of what I want from my theoretical game. It's either going to need generative questlines or an open world. Or both.
In an RPG, the fundamental pacing idea is good. You have several different gears in your gear box: safe exploration, tense exploration, grinding, pushing... each can be entered and exited depending on my mood and my judgment of how my customization is going. Maybe I need to grind a bit more before I push forward, for example.
The choices given to me are not absolutely perfect. There's no maintenance cycle, and the exploration segments are limited by the maps and scripted sidequests available in a region. For example, tense exploration turns into grinding, and no more tense exploration opens up until you push past the next boss. That's actually good in a console/PC game, because it keeps you moving forward. But in a casual game?
Well, there are some games that are a hybrid. The Rune Factory games are RPGs with cyclic maintenance and almost unlimited exploration sections. Huge worlds, low endurance, and lots of NPCs that actually change over time give them a good longevity. But Rune Factory is not what I'm looking to make. The customization is too low, and the worlds are underdeveloped. In addition, the days pass with alarming finality, so it's difficult to adjust a cycle to fit your mood and allotted time. Lastly, the actual interface is not very well suited to phones.
Interface is very important for pick-up-and-play games. Awkward interfaces are fine if you've got at least an hour to let your fingers remember what's going on, but in a commuter game you've got to have an interface that feels really easy. This is true both in how the player interacts with the game, and in how the game presents itself.
RPGs tend to be "local focus". That is, you are an entity in the game world and you can interact with things inside a given radius. But a phone is fundamentally a "global focus" - everything on the screen is as easy to tap as anything else on the screen. This is a big change. For example, a Rune Factory field is full of random plants you've chosen to plant. You plant them by walking to the specific meter of dirt you want and then plopping them down. But on a phone, planting a field would feel more natural if you just tap the meter you want. In turn, this means your fat fingers have to be able to accurately tap a specific meter, which means displaying fewer meters...
Normally, RPGs just accept their sub-par controls and have shitty controls on phones. I'd prefer to go with global control. So there's a challenge to create something where you have global control AND customization AND interesting character development AND interesting world development AND flexible play.
... I don't really have an answer, yet. But I'm working on it!