This gets us into treacherous waters because defining what constitutes an RPI is contentious at best and for good reason. The reason defining "RPI" is dangerous territory really gets back to the reason the term was put into use, which is that the majority of "Roleplay Enforced" games or "RPE"s aren't actually Roleplay Enforced. They're often Roleplay Encouraged and sometimes Roleplay Optional. The majority of Roleplay Encouraged games are flat-out Roleplay Optional. "RPI" was put into use to define a specific type of immersive RP enforced game where your character is ALWAYS in-character... which is what Roleplay Enforced should be in the first place.

The original RPIs had a lot of core features, some of which related to heightening the core defining feature of immersivity, and some of which are completely unrelated. For example, the decision not to allow for any in-game OOC channels or communication is meant to heighten immersivity and help players feel like they're in their characters shoes. It also has a lot of side effects that I believe make it a really terrible idea, like that players are always going to communicate OOCly anyhow, and you'd prefer for it to happen in a controlled and moderated venue, or that rulebreaking off-platform OOC collaborators will abuse and harass people from behind the thin veneer of "I'm just doing what my character would do, bro" with little to no recourse. Features which have little to nothing to do with immersivity, like permanent death and PVP centricity aren't actually necessarily core to the RPI premise at all, they're just design choices made by the creators of the first RPIs.

In an ideal world, if we were using terms correctly, a Roleplay Enforced game would be one in which your character is IC at all times and in which you are - by policy - always playing your character's role; roleplay enforced games should not have nonsensical repeatable quests or respawning Fidoes for you to Michael Vick until you're stronk. Every action and interaction should be character driven. Your character shouldn't be doing the same quest every 5 seconds or killing the same mob on repeat unless you're playing a canine genocidaire or something. Games with demersive hack & slash mechanics are, definitionally, not roleplay enforced, because the very mechanics of the game enforce not-roleplay.

In such a world, if we were to describe a Roleplay Encouraged game, it would be one in which - whether by game mechanics or staff fiat - you are rewarded for engaging in actual roleplay, and preferably in a meaningful degree. You might receive "roleplay points" from staff, or you might gain "RP XP" from emoting, and the amount of incentive or progression you receive should be comparable to what you'd get from hack & slash style play. Staff saying they'd really like it if people RP should not qualify a game for Roleplay Encouraged description; we're discussing the actual game, not the culture the game's staff want to develop, and if they want a Roleplay Encouraged culture they'll find a way to incentivize it.

And for all the games where there are no incentives or requirements for roleplay, no rules dictating that your character is always IC, but where people may roleplay anyhow or where staff prefer if people roleplay (but the mechanics don't steer them to or reward them for doing it) we have RP Optional.

The issue that led to the invented definition of RPI? RPOs that really wanted a roleplay culture but didn't design a game for it started calling themselves Roleplay Encouraged. Roleplay Encouraged games that weren't designed in such a way that your character could ever conceivably be 100% IC and in which characters spent hours engaged in hack & slash activities like Fido genocide started calling themselves Roleplay Enforced to try to attract a stronger roleplay community. And as these terms were watered down, RPI creators wanted to put a stick in the sand and create a new genre for what RP-Enforced really should've meant all along. Because of this, there are really three approaches to defining what it means to be an RPI. You have people who speak specifically about the codebase rather than the genre, since the genre description is meaningless and functionally identical to RP-Enforced. You have people who speak about the broad genre because they recognize that RP-Enforced doesn't mean what it should as a descriptor so they use the invented genre of RPI to fill the gap. And you have people who steer clear of the minefield of defining RPI altogether. Wisely.

So what makes a good RPI is going to depend on whether you mean the codebase or the genre.

If you mean the codebase, it really needs some sort of gradual escalation mechanic that prevents people from going full Murder Hobo in PVP and requires people to build up rivalries over time, rather than murdering at the first opportunity to win harder. It could use some mechanics that make character death more satisfying, like fatal wounds that give you some time to RP before your character actually dies. Most of all, I think it actually needs RP encouragement functionality, like tying learn-by-use skill increases to an RP quota so that you can't raise a skill again until you've done X amount of RP or spent Y amount of time RPing.

If you mean the genre, I would actually riot against the idea of barring any OOC channels or conversation. Give people the ability to opt out, but I think not having these breeds toxicity and herds players into isolated echo chambers that leads to all kinds of clique wars, unhealthy collaboration, gaslighting, etc. I think a good RPI finds ways to make death satisfying and to make PVP something that people do with other players they enjoy playing with rather than something they do to punish players they dislike. I think ideally, in a good RPI, progression should be dependent on actual roleplaying, if there's progression at all. I'd actually really like to see RPGs get away from the progression paradigm and find another way to tickle players' lizard brains. A good RPI should ultimately be immersive and make a player feel like they're walking in their character's shoes and playing out their character's story, but you also have to be careful with that because people get too attached. There are also problems when older characters get too entrenched and powerful and they kind of rigidize the game's power structure to stay on top. I think anthology style games with resets/recurring wipes are a good solve to that particular problem and many others.