Presenting for your consideration:
Time
(It's a little-known fact that time has a refreshing citrus flavour)
I think Time has the potential to be a really interesting and rare type. Not only does it have at least one associated Legendary in Dialga, it also can cover some more esoteric choices with 'mons that don't really fit into a particular pre-existing type, such as Sigilyph and Golurk. There's also the potential link with Fossil Pokémon - indeed, one might almost be tempted to give the Time typing to alternate versions of Fossil mons, ones that weren't reanimated by probably-passes-for-science-in-a-poor-enough-light but were instead dragged directly to the modern world from all those millions of years ago.
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Type Effectiveness Ideas:-
Offensive:-
Super-Effective vs:
Dragon,
Fighting,
Fire,
Heart,
Steel
The dragons of the world set great store in their perceived immortality; across the span of aeons, time proves that thinking to be the madness it is. You cannot fight the march of time no matter your heroics. Hearts will still, iron will rust, and fire? Fire is but a flash of light in an endless sea of dark.
Not Very Effective vs:
Bug,
Grass,
Ground,
Rock
Some forms of life adapt to the presence of time, be it through the intense swiftness of insect life or the long acceptance and seasonal change of flora. Time's sting changes little when it comes to stone, though.
Ineffective vs.: --
Defensive:-
Resistant to:
Dragon,
Electric,
Fighting,
Grass,
Heart
The rage of dragons, heroes, and similar brave and caring souls is doubtless great, but against the armour of time they are as falling leaves. Leaves, too, struggle to damage creatures of Time; while they adapted to its caress, that is a defence, not an attack.
Weak to:
Ghost,
Poison,
Psychic
Beings that exist outside of the normal constraints of time - planar travellers like Psychics, the restless dead - can penetrate Time's armour, but Poison is perhaps the most surprisingly effective, turning Time's own weapons of entropy and decay against it.
Immune to:
Fire
No matter what ancient gods may tell you, fire fades and ends. Its power is naught against the infinite scope of Time.
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Potential retyped Pokémon:-
Fossil mons (alternate formes)
Omanyte line: Water/Rock --> Water/Time
Kabuto line: Water/Rock --> Water/Time
Aerodactyl: Rock/Flying --> Time/Flying
Lileep line: Grass/Rock --> Grass/Time
Anorith line: Bug/Rock --> Bug/Time
Cranidos line: Rock --> Rock/Time
Shieldon line: Rock/Steel --> Time/Steel
Tirtouga line: Water/Rock --> Water/Time
Archen line: Rock/Flying --> Time/Flying
Tyrunt line: Rock/Dragon --> Time/Dragon
Amaura line: Ice/Rock --> Ice/Time
NB: This is not calling for them all to be retyped. Rather, it is a call to analyze how (and indeed if) they would be changed were they to possess a Time typing. These would work better if, as I said above, these were different formes available through something like a Reversion Machine rather than blanket retypes of the species.
Dialga
Dialga: Dragon/Steel --> Dragon/Time
Come on, guys. It's the Dragon of Time.
Artifact Pokémon
Unown: Psychic --> Time/Psychic
Baltoy line: Ground/Psychic --> Ground/Time
Sigilyph: Psychic/Flying --> Time/Flying
Golett line: Ghost/Ground --> Ghost/Time
These are artificially-created Pokémon that have ties to ancient civilizations in both design and lore.
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Potential retyped moves:-
Physical: Bide, Extreme Speed
Special: Echoed Voice, Roar Of Time, Swift
Status: Destiny Bond, Double Team, Encore, Foresight, Metronome, Trick Room
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Move design ideas:-
Amber Pulse | 85 BP | Special | 100% | 10-16 | Pulse | No contact | Priority: 0 | All enemies | No additional effect | Z: 160 BP
Warp Strike | 90 BP | Physical | 90% | 10-16 | -- | Contact | Priority: 0 | Single enemy | 30% chance to lower target's Evasion by 1 stage | Z: 170 BP
Future Shock | 40 BP | Special | 100% | 20-32 | Pulse, Bomb | No contact | Priority: +1 | Single enemy | No additional effect | Z: 100 BP
Echo Strikes | 20 BP | Physical | 90% | 20-32 | -- | Contact | Priority: 0 | Single enemy | Hits 2-5 times | Z: 100 BP
Pressure Wave | 30 BP | Special | 100% | 10-16 | -- | No contact | Priority: +1 | Single enemy | 20% chance to lower two of the target's main stats by 1 stage | Z: 100 BP
Disrupt | 60 BP | Physical | 90% | 5-8 | -- | No contact | Priority: +2 | Single enemy | This move's Base Power is doubled if the user moves last. Hits through Protect | Z: 120 BP
Reinforce | -- | Status | -- | 10-16 | -- | No contact | Priority: +1 | Self | Switches the user out. Raises two of the incoming Pokémon's stats by 1 stage | Z: Heals the user to full before switching them out
Rewind | -- | Status | 100% | 5-8 | -- | No contact | Priority: 0 | Single enemy | Resets the target's stat adjustments. If a stat is affected, the user heals for 50% of its max HP
Preservation | -- | Status | -- | 5-8 | -- | No contact | Priority: +1 | Self | Raises the user's Defence by 2 stages. Raises the user's Special Defence by 2 stages if it moves last
Forgotten Power | -- | Status | -- | 5-8 | -- | No contact | Priority: 0 | Self | Raises the user's Attack or Special Attack by 2 stages, whichever is higher. Raises the user's critical hit rate by +1
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Thoughts? =]