1.   trainer  battles

After over two years of anticipation, Pokémon GO is finally adding the ability to battle other Trainers! The feature is detailed in depth here, and will launch this month (as soon as this week, in fact).

A new update has begun to be released, which brings support for this new feature. It is not yet live, as the update has not come to all devices and been forced yet. The update also brings increased avatar customization, a new loading screen, and also some assets that have no known purpose.
SOURCE: Niantic, IGN, Chrales

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2.   HOLIDAY  TEASE

The latest Pokémon GO email newsletter teased the annual Holiday Event in its final sentences. We don't know much about the event, other than the fact that Delibird will once again be available.
SOURCE: Pokémon GO Newsletter

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3.   RESEARCH  CHANGE

On December's Community Day, research underwent its monthly shakeup. Shedinja was replaced with Articuno, Moltres, Zapdos, Entei, Raikou, and Suicune as the Research Breakthrough encounter. The Legendary Birds are able to be Shiny, as is the newly-added Shiny Misdreavus. A new Spinda variant has also been released.
SOURCE: Niantic, Community Reports

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4.   POKEMON  DUEL

A new update has come to Pokémon Duel, making it the first substantial update in some time. This update teases the new Z-Move feature coming in 2019 by releasing Bloom Doom, Hydro Vortex, and Inferno Overdrive.
SOURCE: Serebii

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5.   COMMUNITY  DAY

Last weekend was a multi-day Pokémon GO Community Day. All eleven of the past featured Pokémon were back with increased spawn rates, Shiny chances, and the ability to learn their exclusive moves once again. The next Community Day has gone unannounced.
SOURCE: Niantic