While having recovery to incentivize Diamond Storm usage through getting long-term value out of defense boosts is extremely important, I also think some practical recovery from a defensive perspective will also be pivotal. Regardless of what role CAP 31 takes, having access to longevity not only means more turns to come in against things like Zeraora, Tapu Koko, and Heatran, but by extension, more long-term leverage to come in against Pokemon it wants to check means more turns to boost or make progress via other means. Longevity plays into both ends of the spectrum in a way that really sells its niche; it lets it play as a consistent win condition as well as provide constant defensive utility over a longer game, both of which are noteworthy in somewhat similar ways.
That then leaves what kind of recovery to give it. I believe there are two very strong and good options that I would like to advocate for, but I'd first like to explain the issues with some of the contenders.
Leech Seed: A massively appealing idea on paper, considering it provides a form of passive recovery that lets CAP 31 continuously boost without having to waste turns healing. In practice, though, you ought to have no recovery at all considering what Pokemon CAP 31 is going to be finding itself against, and how it would be playing against them. Ferrothorn as a Leech Seed user works mostly because it has access to safe utility options to stay in and keep farming HP, and because it packs great natural bulk and the typing to force switches and easier recovery. The issue I foresee is that we'd have to follow heavily in Ferrothorn's example to really make Leech Seed work, which means taking up moveslots and BST to really reap value out of Leech Seed and consistently make progress against its pivots without just being a Leech Seed bot. I would think that's even more of a problem since CAP 31 is already wanting two offensive moveslots, and that Leech Seed immunities already threaten it pretty hard. It could work in theory, but I worry that it would either mean having to center CAP 31's gameplan around Leech Seed to a point where the value of Diamond Storm is going to be rendered down a lot, or just foregoing it altogether.
Wish: Somewhat of a similar reason to Leech Seed, but a bit more intensely since it would mean having to run Protect to simulate automatic recovery. CAP 31 can't afford that considering it's already strapped for moveslots and I'm 100% certain we would want at least one of them to provide something to diversify and/or augment CAP 31's matchup into Pokemon that block it from doing much otherwise.
Strength Sap: My main issue with this move is that it entirely downplays a core appeal of Diamond Storm since it compresses the idea of dampening physical blows and recovery into one move, which really plays into the idea of diminishing returns as stated, and I would argue is anti-concept. Not too much else to say here.
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The two options I am most on board for are Pain Split and 50% Recovery Moves.
Pain Split is easily my favorite recovery option here, and there are a few reasons why. The biggest reason, though, is that CAP 31's matchup spread provides an incredible complement to Pain Split's design. What I mean by this is that, as a Ground-type, it's going to be intrinsically tasked with taking on a lot of really important moves rather consistently throughout the game, such as likely being a dedicated Knock Off pivot, a Magma Storm switch-in, and being the recipient of many U-turns by nature of its natural defensive profile covering so much ground. This will mean that Pain Splits will be, on average, higher value just because of how much pressure it's going to face (Rotom-W is a good example of this in action, base HP aside). Furthermore, I think it's important to consider how Pain Split's damage interacts with CAP 31's natural checks, especially Corviknight which it will have an extremely hard time playing against for reasons we've previously explored. Being able to simultaneously heal and force chip against it without having to dedicate a moveslot into pressuring it directly feels so huge; even just forcing a Roost turn without having to waste Diamond Storm PP can be critical in making progress. Diamond Storm's defense boosts are also a nice complement here since they leverage CAP 31 to be at a lower HP threshold, and then force crispy damage/healing simultaneously. It makes me really giddy thinking about the kinds of interactions Pain Split could really have on this CAP; it feels like such a strong complement to what we've built so far, and even better is that it gives us room to play around with its movepool/stats due to being intrinsically inconsistent.
50% Recovery I also like. In short, reliable recovery is good! CAP 31 becomes a Ground-type with great recovery and can consistently/safely play to its boosting to really win out longer games while maintaining its defensive utility for teams in the long run. We've already established what makes this option as solid as it is, so instead I'd like to talk about the differentiation between the limited and strong versions of recovery.
Option 1: Shore Up, Recover, Slack Off, and co. are obviously, the stronger of the options, mostly due to sheer consistency and a great PP pool, which can really make a difference in longer games. To me, I'm much more concerned about PP; with this option, CAP 31 has the ability to last effectively twice as long, which I think is the key point to consider. Do we value CAP 31's long-term defensive utility as being the most pro-concept? If so, I think this option is the wave. It is a far greater complement to a much more sturdy blockade that reinforces teams, but also lets it still play to Diamond Storm through, simply, being reliable recovery for us to reap value from as a bulky win condition.
Option 2: Synthesis, Morning Sun, and Moonlight have merits too. Although they are definitely the weaker options, them being weaker through a reduced PP pool means that CAP 31's usage of recovery needs to be, on average, more deliberate, especially if it's being designed as a win condition. It will need to be more sparing with its PP when it's out to deploy its boosts since it will be certainly taking a lot of heat, also in part to how it will likely be using PP in the process of checking Electric-types and Heatran regardless. The primary appeal here is that their lowered PP pool and weather inconsistency leverages the stat and movepool stages a bit more, which is of note.
I personally value Option 1 more, mostly because I don't really see much value in limiting 50% recovery if we're decided on that as our route. It'll still have the same interactions, and if anything, I think optimizing room for CAP 31's defensive utility to shine in tandem with Diamond Storm boosts as a win condition are, most certainly, pro-concept considering how CAP 31 is designed around taking advantage of its defensive profile to accumulate boosts in the first place. The difference, in practice, is pretty small, but I think it's definitely worth considering.