I don't get why we have to "clear up" the lower rankings. There are a lot of not-so-great Pokemon that still have a small niche and the viability rankings should represent that. We shouldn't have an obligation to remove our least favorites off of the viability ranking thread if they're not that bad, and in my opinion Milotic is not bad enough to not be represented at all.Really we just cleared up the lower rankings and now you are suggesting extremely niche Pokemon again.
Honestly I like Defensive Meloetta. Also note that (on offensive Meloetta) Relic Song is still STAB and can hit certain Pokemon hard, not to mention using it reverses Meloetta's checks and counters. Its niche is pretty small, but it performs it well enough to be higher than D, I think.
Obviously Trick Room isn't the best playstyle in the world, but it's not like it was in previous generations either. Almost all offensive Trick Room setters have good coverage that allows them to beat certain Pokemon that would be considered their counters and checks, and defensive ones at least cripple their checks and counters before switching. If certain Pokemon that need or greatly appreciate Trick Room to function are sitting higher up in the rankings, then why not give some credit to the Pokemon that support them as well? Sticky Web and Trick Room are not the same thing at all. After a Sticky Web, Pokemon with around 70 or 80 base speed start to outspeed things. With Trick Room Pokemon with 10 to 60 base speed outspeed things, not to mention that Sticky Web boosts Bisharp's attack or doesn't affect Flying-types. Trick Room and Sticky Web are very different, IMO. Sticky Web teams often only have a single setter, while Trick Room teams are usually half setters. There also aren't very many teams that don't revolve around Sticky Web but still use it, while Trick Room setters that just support one or two Pokemon aren't unheard of. Also Hyper Offense is still considered the best and most popular playstyle, and just because Aegislash and a few megas were introduced doesn't mean that frail Pokemon don't exist.
The point of Cofagrigus is to be a good, bulky, defensive Trick Room setter that can still deal damage if needed. Very few other ghost-types are slow and bulky and have access to Trick Room. Also, currently it's D not unranked (and it has to be ranked because it has an analysis), and saying that every physical attacker has Knock Off is an overexaggeration. Perhaps C- is the place for it and Reuniclus, but wherever, it shouldn't be D, nor unranked.
Gourgeist small has a bad attack and high speed. It acts as a quick staller with Leech Seed, Will-O-Wisp, and Substitute, not as a Trick Room suicide lead. Their niches are completely different, so there's no point in comparing them.
I've never heard of a team that was so weak to Volt-turn that it needed a dedicated Pokemon to counter them. Besides, most Volt-Turn Pokemon can at least 3HKO Gastrodon, so it's not going to be walling their whole team, especially not the parts of their team that don't carry U-turn or Volt Switch.
Snorlax was worse in generation 5, not better. Snorlax never runs curse anyways, the only set I've ever heard of is Assault Vest. 252+ Atk Snorlax Crunch vs. 252 HP / 0 Def Aegislash-Shield: 122-144 (37.6 - 44.4%) -- guaranteed 3HKO after Leftovers recovery. Sure sure, piss weak non-stab. And that's Shield form, even though Snorlax is slower and will therefore have a high chance of OHKOing it. Not to mention Earthquake, which can 2HKO shield form and OHKO blade form. Snorlax is not too hard to wear down due to it's lack of recovery, but the same can be said for many bulky Pokemon. However I'm not the person to argue for this, really. Snorlax just seems way better than a D rank Pokemon, as it's not just a niche Char-Y counter and it seems alright.
There are two Pokemon on rain dance teams with an electric immunity. Seismitoad and Thundurus-T. Seismitoad is faster but weaker than Thundurus-T as the only move that it can use that is powerful is Hydro Pump, as it's coverage moves don't really do much at all. Thundurus-T has powerful Thunder as well as powerful coverage, as well as a grass resistance and an ice weakness, meaning it will usually have better synergy with the rest of the team. That's not to say that Seismitoad is bad though, it's pretty good, but it doesn't really outclass Thundurus-T, nor does Thundurus-T really outclass it that much.
Mantine and Gyarados and Swampert and Quagsire don't necessarily compete for a teamslot. They all perform fairly different roles, but you wouldn't use them on the same team together. And Gyarados will be asleep just as much of the time as Mantine. Gyarados checks many Pokemon, Mantine hard counters a few specific ones, mainly Charizard Y and Landorus. (Now Swampert) In my experience phazing with hazards is more reliable than forcing switches while hazards are out because then your opponent gets to pick and the Pokemon that you're trying to force out could very likely have a coverage move to nail you, although you probably have Protect or something similar as well. I've never heard of a defensive team without hazards. Also Skarmory doesn't have mixed bulk, which IMO most phazers should have since you don't want to be switching out after every other Roar/Whirlwind.
Empoleon's secondary steel typing does almost nothing for it. It gives it two extra weaknesses and turns a resistance into a neutrality for a Dragon resistance (amongst more unimportant resistances) and a neutrality to Grass, which isn't a very common attacking type anyways. I guess it sort of has a niche as a check to certain Pokemon such as some Dragon-types, but that's done better by other Pokemon anyways. Its Steel STAB is unneeded and doesn't give it much more coverage.