OU ARCEUS
Description
A pokemon that, like Arceus, changes its type based on a certain factor. This means its STAB could be any type based on said factor.
Justification
In the OU metagame, pokemon usually serve only one purpose, and therefore are extremely predictable. An example of this is Scizor. The majority he is brought on, he is Choice Band and is going to Bullet Punch / U - Turn, which makes him extremely easy to predict and switch to the appropriate pokemon. Another example of this is Salamence. He is almost always one of two things; Mixmence or DDmence, and both carry very similar counters. A pokemon that is unpredictable type-wise would add another dimension of prediction to the game.
Questions To Be Answered
1) If an extremely unpredictable pokemon is put into the metagame, will over-centralized sets become less used in favor of unpredictability?
2) How would walls such as Blissey or Skarmory deal with the presence of a sweeper who could hit unpredictably with a certain attack type or certain attack spectrum?
3) Would a pokemon with this chance to have any STAB start to have its own predictable set?
4) How will the metagame change around to combat a threat that could be anything
Explanation
Scizor, Weavile, Salamence, Metagross, Suicune, Vaporeon, Blissey, Skarmory, Infernape, and Lucario are just some of the examples of pokemon that are used for the same thing almost every time. Scizor, the CB Technician abuser, Weavile, the revenge killer, Metagross, the suicide lead/late game Agili sweeper, etc. These pokemon can thrive despite being overly predictable in the OU metagame because almost everything that counters them is also extremely predictable, (Cresselia for Salamence & Metagross, Heatran for Scizor, etc.) and therefore they have partners to take down these things that stand in their way. A pokemon that couldn't be predicted until the majority of the set was revealed would greatly increase the amount of mind games that occur in the metagame.
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Example A.
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Salamence forces a Choice Banded Metagross locked onto Earthquake out. (This happens often)
Salamence uses Dragon Dance. (Of course.)
CAP 9 comes in. (If CAP 9 hadn't been revealed up until this point in the battle, Salamence has no way of knowing whether this version of CAP 9 is tailored to defeat him, or if its something that he could beat easily, oh the mind games. This means that he can't risk an attack. If he uses Earthquake, they might be Flying, he can't risk if CAP 9 is Steel and get locked into Outrage, etc.)
+1 Salamence switches out.
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Now if they switch out, they still have no idea what the type CAP 9 is, so they will have to take some risks in order to find out.
This is just one example of how, if CAP 9 went in this direction, overly centralized sets such as DDmence would have to be on their toes to be successful.
Hope you like the idea, if there is anything illegal, please tell me and I'll fix it.