It's already been voted on. Drizzle remained OU because banning it would cause severe imbalance in the metagame, which is why an exception was made for weather banning abilities.
I'm aware that it has been voted on, it can be nominated and voted on again if Deoxys-N was any indication.
Notice the words I bolded for you. Why didn't you say that banning it caused severe imbalance with certainty? Because you don't know. You don't know, I don't know, the community at large simply DOES NOT KNOW whether or not this would be the case, which made this a preemptive ban of sorts.
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Anyways, we've moved on from "what is broken and what isn't" to "what will create the healthiest, most balanced metagame possible?" If looked at from this perspective, here are some things to take note of.
How Manaphy Helps the Metagame
If we were to do something like this in the first place, we would need to justify it by showing that Manaphy does in fact help the metagame, and I believe that it does based on past rounds as well as theorymon.
As it stands, the water-type received a pretty big nerf in the form of Ferrothorn, which was a good thing because they have been virtually unchecked in the past by virtue of the grass-type's issues. In addition, the metagame in general is simply less kind to them than it has been in the past, there being several examples. Crocune isn't nearly as effective, LO Starmie isn't as effective (and is forced to run HP Fire if it hopes to be), Gyarados is now relegated to the role of bulky water and thus has to use ResTalk, Vaporeon remains about the same.
The grass-type in general has received various boosts, the first being Ferrothorn, who is capable of fitting on nearly any team and provides adequate team support while being able to actually do something in return. Another is Breloom, who gets a more powerful Bullet Seed and is poised to become pretty popular in general if the DW tier is any indication. Virizion's claim to fame is essentially taking down rain teams on its own with its high special defense and decent offenses.
The metagame is largely centered around types that hit water pokemon for neutral damage and have high base power moves, meaning that their raw bulk just isn't cutting it - I'm talking about Dragon and Fighting. Blaziken, Conkeldurr, Mienshao, Lucario - all are powerful fighting types that can simply muscle their way through bulky waters with ease, and the same goes for dragons in the form of SpecsLatios, Garchomp, Salamence, Hydreigon, and more. Water is still good, but the challenges it has always faced now surface more than ever.
Manaphy could help out with several of these issues, checking a decent bit of the metagame assuming Max/Max Bold + Surf / Calm Mind / Rest / Rain Dance.
-A solid check to Jolly Balloon Excadrill, taking (60.40% - 71.29%) from +2 Earthquake and retaliating with (80.11% - 95.03%) from Surf.
-A solid check to Adamant LO Rock Polish Landorus.
-A great check to Volcarona, as it can take it down with Surf if its weakened enough or throw up rain to nullify the threat level to another team member.
-Top Heatran check
-Good Conkeldurr check, as it can throw up rain and start firing boosted Surfs.
-Provides the option of immediate special sweeping boosting threat of the water-type. The closest we have to that now is...Simipour?
-A general check to
all weather, as it carries a weather move of its own and uses Drizzle's weather to its advantage. This is what we were looking for in keeping Drizzle, right? If manaphy can do the same thing...then why is one Uber and one forced OU?
The list goes on. Remember that these are assuming Max/Max Bold, so Manaphy can EV itself as necessary depending on what you need it to check, and you could even run a TG / Rain Dance / Surf / Filler if you don't want Manaphy to be purely a pivot.
So the next question is, can the metagame handle Manaphy outside of eternal rain? Yes, here's how it can be done (some theorymon included):
-Latios and Latias: Top checks to Manaphy thanks to their bulk and access to power electric moves or Recover + CM. Lack of Ice Beam means Crophy is out of luck.
-Ferrothorn is a great defensive check to Manaphy, hitting it with Power Whip as Manaphy is forced to employ the RainRest cycle on its own due to lack of support.
-Virizion / CMCune / Vaporeon / Bloongell / Water Absorb will stop Crophy cold, while TG Manaphy lacking RD will be affected by status as a consequence. 2-move coverage Manaphy will be walled by common pokemon either way.
-Powerful neutral hits. Water has great special resistances, yes, but rarely has the bulk to take stronger neutral hits. LO Terakion CC, SpecsLatios Draco Meteor, etc, will all take their toll on both forms of Manaphy.
There are various options. In any case, I believe Manaphy benefits OU without breaking it so long as permanent rain is not involved, which is where the clause would come in.
And stop claiming that no Drizzle + Manaphy nerfs Manaphy. Nothing about Manaphy is changed, its the environment that it plays in, which is, again, the same thing we've done to the swift swimmers.