Pigeons
pidge pidge
I absolutely disagree with a Grassceus rise, I'll try and address everything you said though.Grassceus is really underrated. Tbh it's one of the best leads in the game due to how it counters smeargle and can do more damage than other arceus forms with grass-knot. This also sounds like a mega meme but it can run roar of time to OHKO mega rayquaza (yeah, it sounds awful but somehow it got me to #2 in 2050s). Calm mind grassceus with flamethrower and/or ice beam is also p good, destroys ferrothorn and celesteela which are absolute AIDS. And plus it has arceus's superb bulk with base 120 stats across the board. Grass isn't really that bad of a type. People tend to think "oh, it's not that good because primal groudon and mega ray are both big threats to it," but it can still heavily damage both of them, as well as outspeed them (unless ray is scarfed). Steel types also aren't an issue for it if it has flamethrower. Honestly I just think that being the only arceus form with immunity to spore and leech seed combined with the other things that I mentioned make it deserving of a rise. Also to note, if you look at the "viability ceiling" of grassceus in the moveset statistics it is at 96, which is the highest viability ceiling for August along with some other pokemon. Comparatively, xerneas (which I'm personally not a fan of) is at a ceiling of 95 despite being ranked much higher on the viability rankings.
Firstly, Grassceus is currently ranked in B-, so if you're going to nom something up it would help if you gave us an idea of why Grassceus is better than Pokemon it shares a rank with such as Gothitelle and Magearna rather than giving an overview of what it does (most people are already aware of this).
I definitely wouldn't call it one of the best leads in the game. Lead Grassceus does have a nice Spore immunity for dealing with Smeargle, but vs most dedicated leads it has a rough time without Magic Coat. Magic Coat does give it the edge against leads like Smeargle, Shuckle and Deoxys-Speed but Spore immunity alone isn't enough to set it apart from other Magic Coat lead Arceus (Fairyceus for example). Losing to PDon and MRay in a lead situation isn't great either because these Pokemon are often used as leads as well.
Magic Coat also highlights another one of Grassceus's issues: 4MSS. I know that word gets thrown around a lot, often when it isn't justified, but in Grassceus's case it really holds true. Grass Knot is obviously required and you don't want to be dead weight against Rayquaza, meaning Ice Beam or a Dragon move is pretty much a necessity as well. That leaves 2 slots for Flamethrower / Fire Blast to do something vs Steels, Stone Edge to not be Ho-Oh bait, Recover to have some degree of longevity, Stealth Rock to have some kind of team support or Magic Coat to act as a lead. Whichever two you choose, Grassceus will have some matchups where it is severely lacking.
Immunity to Leech Seed is definitely one of Grassceus's biggest selling points, and Spore immunity is quite nifty too, but you aren't really mentioning the downinsides of its Grass typing. Even though it can threaten them to some degree, Ray, PDon, Ho-Oh, Yveltal are all common offensive mons that put a ton of pressure of Grassceus by virtue of typing alone. Grassceus fails to 1hko these threats (Roar of Time is seriously bad and I don't feel like discussing why) so unless they have some prior chip, Grassceus will always be losing momentum vs some of the most common Pokemon in the game, or simply dying. Whatever its purpose on a team is, getting trapped by MGar is a huge issue for Grassceus that again stems from its typing.
Just for clarification to others who don't know what "viability ceiling" is, it refers to a maximum GXE achieved on the ladder with a certain Pokemon. I don't think this is really a major factor to consider when ranking Pokemon given that it is more indicative of the preferences of skilled ladder players than the actual merits of Pokemon themselves. Ladder typically isn't the best judge of viability to begin with given the generally low quality of players there.
Grassceus definitely has some nifty aspects to it, shutting down Ferrothorn and Celesteela is quite nice given how much of a pain in the ass those mons can be, Grass typing also has some merits you didn't mention like resistance to Zygarde's Thousand Arrows and being an alright soft check to POgre. That aside, Grassceus has way too many flaws to rise from a rank that is very generous to begin with, especially when looking at how powerful some of the Pokemon in B are (see Chansey and Giratina).
TL;DR: Don't rise Grassceus
I realize I never replied to this so here we go again. I think you're defining viability exclusively in terms of splashability which isn't really the whole picture. If you look at other Pokemon in B and higher, you'll notice Chansey isn't the only one largely restricted to one playstyle. Sableye is almost always found on stall or extremely bulky balance, Deoxys-Speed and Deoxys-Attack are exclusively found on hyper offense, etc. We can argue about the details of this analogy and but the fact remains these Pokemon are similar not terribly splashable but remain highly ranked regardless, so the amount of playstyles something fits on is clearly not the only factor in viability.The fact that Chansey can ONLY belong on stall makes it extremely niche, in addition, it is outclassed by the likes of ferrothorn, xern, ho-oh, pdon, and pogre in terms of the roles it can fulfill on most teams, thus making it require massive team support in stall mons. Thus, I still stand by my nom that Chansey should get dropped.
The point is, you also have to consider how well it performs its given role. We argued about team support already, but the Pokemon you listed as "outclassing it" don't outclass it in the slightest. Xern and POgre are offensive Pokemon with no similarity to Chansey so I have no idea why you brought them up, the others can perform marginally similar roles in checking some special attackers but the similarities end there. Ho-Oh is an offensive tank, Ferrothorn offers Stealth Rock and Leech Seed support, PDon also offers Stealth Rock support and greater offensive presence. Compared to these Pokemon, Chansey is a check to just about every special attacker in the metagame that can also provide cleric support, a massive boon on stall teams and some balances.
Chansey is practically mandatory on stall and an amazing edition to certain balances, requiring team support is a factor in viability for sure but you shouldn't overlook the huge amount of team support Chansey is able to offer in exchange. In my opinion, Chansey doesn't require a greater deal of team support than say, MMY or Giratina, this combined with Chansey's huge overall utility leads me to believe Chansey should stay B.
TL;DR Don't drop Chansey
Some other noms:
Yveltal's been a lot harder to use nowadays, it just doesn't fit on teams like it used to and teams generally aren't that weak to it. Fairy-types are more common and typical bulky offense teambuilds that dominate the ladder aren't really bothered by it at all. It's still pretty good but just a lot harder to fit onto teams than fellow A-rank members like Zygarde and Celesteela and meta trends really don't favour it much. Could honestly drop farther but it seems like a decent fit with POgre and other A- mons.
I know it just rose but it seems like Goth just keeps getting better. The Charm + Confide + Taunt set that Thimo introduced has made Goth all the more annoying to face, as now it can trap stuff like Ferrothorn or Defensive Zygarde lacking DTail. The prevalence of balance is great for it, as against such teams Gothitelle is often able to get multiple kos and open massive holes in the opposing team. The previously mentioned Charm + Confide set doesn't just remove threats though, it can turn them into setup bait for just about any threatening sweeper after their offensive stats have been reduced to -6. Gothitelle really stands out in B- because no Pokemon there puts in the amount of work that it does while also having so little counterplay. Even though Gothitelle has issues against faster offensive teams, I think the massive pressure it puts on bulkier teams warrants a rise.