Alright, just indexing the current "Pending" list [Eruption, Brine, Charge Beam, Dark Pulse, Power Gem, Mud Bomb, Mud Shot, Hex, Shadow Ball, Signal Beam] against the agreed-upon threat list, I'm comfortable with everything here beyond Brine being
allowed. I'll go through case-by-case here.
Eruption is a rather comical case of interaction with Dry Skin. Tt can get a vicious 150 BP STAB sun-boosted hit off... once. After that Dry Skin kicks in and it drops to 132 BP, which is still rather formidable. Given another shot—which is either against the weakened target of the first hit or against a revenge killer, assuming Mollux stays in—and it drops again to 114 BP, now losing to Fire Blast. Most residual damage and healing can be roughly measured in Eruption BP loss, actually: Stealth Rock cuts 37 BP, Sandstorm cuts 9 BP per turn, Dry Skin gives or takes 18 BP, and so on. Given a spinner, a wish-passer, and a bulky switch-move user to get Mollux in, then a Choice Specs 150 BP Eruption is physically possible. Just highly improbable. More importantly, the weather that boosts Eruption weakens Mollux and vice-versa, so both weathers are detrimental to Eruption. This means
Eruption encourages weatherless Mollux, a concept we really haven't explored. For good reason, I imagine.
Brine is cute, even if the pitchforks are out against it. Against the majority of our threatlist (specifically the ones without Rock typing), Brine scores a weaker hit than Fire Blast (130 BP for SE, assuming high target HP, vs. Fire Blast's 180 BP). This changes up a bit in the rain, and would be the main reason to exclude Brine. Brine really does dick-all to the things we want to threaten (except O'Heatran), but tears in to our counter list like no other. At least, not like anything that's currently allowed.
Charge Beam is a cute option that can act as a watered-down option to Acid Spray, except for one key difference in that the boosts are only lost if
Mollux switches out. Picking up a stray boost could put foes in a very tight spot, but Mollux can only squeeze a Charge Beam boost in 70% of the time, if the foe doesn't switch to Landorus or another Ground-type. There's no real harm in having this, unless creating a SubCharge set counts as "harm".
Fiery Dance deserves mention here, since it's roughly the same deal (60 BP after Rain and STAB) with a weaker boost chance, but becomes a powerful tool for punishing a weather change with or without boosts. It also, notably, has identical BP and typing to Lava Plume, which was the reason our 131 SpA stat was chosen—those rainy KOs on Scizor and Ferrothorn still stand whether it's Lava Plume or Fiery Dance.
Mud Bomb and Mud Shot are rather double-edged. On one hand, there's no better way to scare opposing Mollux, short of H.P. Ground. On the other, the coverage on the Rock half of our threatlist is off-putting. Mud Shot seems safer if we err this direction, and it's not touching something like BalloonTran or Dragonite any time soon.
Hex is cute with the Burns we're spreading, but we're getting less damage from SE boosted Hex than we are on something like Thunder or Fire Blast on just about everything he'd be trying to Hex. We are a powerful status platform as it is, though, with Thunder and Lava Plume flying around—we inflict a status every third attack on average. No point in disallowing it, but there's reason to have it.
Power Gem is interesting, giving us a free H.P. Rock that we're liable to never use. Dragonite takes roughly the same hit from STAB Sludge Wave and is more afraid of H.P. Ice, Thunder, or Lava Plume. It nails opposing Mollux, giving it reason to appear in the movepool, but we're not likely to employ it.
Shadow Ball, Dark Pulse, and Signal Beam don't do much for Mollux at all, but I'd like to see a lot of types in the movepool. They're no scarier than anything else we can carry, really. There's no reason to disallow them if someone like-minded wants to include them.
If Brine and Mud Bomb got axed and everything else here got in, we'd be liable to employ Hidden Power
Ice, Water, or Grass. I can't really put together any potentially overpowering sets with that. I can see, perhaps, Thunder / H.P. Ice / Lava Plume, or Fire Blast / Sludge Wave / H.P. Grass. Substitute / Mud Shot / Thunder / coverage is an option in Rain to slow down and overtake counters, but this specializes Mollux too far, as far as I can tell.
edit: ooh Hurricane hmm
[box]252+ Mollux Leftovers Hurricane to Specially Defensive Tyranitar: 8.66 - 10.14%
252+ Mollux Leftovers Hurricane to Choice Band Tyranitar: 12.63 - 15%
252+ Mollux Leftovers Hurricane to Choice Scarf Landorus: 60.81 - 71.78% (guaranteed 2HKO)
252+ Mollux Leftovers Hurricane to Offensive Heatran: 21.05 - 25.07%
252+ Mollux Leftovers Hurricane to Specially Defensive Heatran: 14.8 - 17.4%
252+ Mollux Leftovers Hurricane to any Terrakion ever: 32.71 - 38.27% (99.39% chance to 3HKO)
252+ Mollux Leftovers Hurricane to Defensive Gliscor: 51.97 - 61.29% (58.98% chance to 2HKO after Poison Heal)
252+ Mollux Leftovers Hurricane to 4/0 Dugtrio: 91.5 - 108.01% (50% chance to OHKO)
252+ Mollux Leftovers Hurricane to Defensive Politoed: 37.5 - 44.27% (guaranteed 3HKO)
252+ Mollux Leftovers Hurricane to Choice Scarf Politoed: 44.85 - 52.95% (28.13% chance to 2HKO)[/box]
Yeah I don't see an issue with allowing
HURRICANE!!! for Mollux. Taking out Toxicroak is fun little side-effect.