Maxis' Redwood Studio is currently hard at work on The Sims 4, but apparently they have something else in the pipeline, as revealed by a career opportunity ad seeking a Senior Designer:

Here's the whole text of the ad with the relevants parts bolded:

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Maxis is seeking a Senior Designer to own and design many critical systems for a new, unannounced Maxis product being created at EA’s Redwood Shores, California Studio. As a Senior Designer, you are responsible for the innovative design of new game systems in this unique, emerging IP, working with other designers to create a consistent, unified design.

Working closely with the Game Director, Executive Producer, Artists, Engineers and Producers, the Senior Designer will define and flesh out the game’s core systems, including mechanics, player goals and drivers, creativity editors, gameplay progression, extensive multiplayer gameplay mechanics, in-game economies, technology trees, HD, PC, and mobile and tablet companion application integration, and essentially all other systems in the game.

The Senior Designer facilitates and builds upon the game direction by driving the design toward the key creativity and social pillars of the product, pushing for development of uniquely innovative gameplay mechanics, and creatively leveraging technology, art, and audio to expand the design into an immersive experience.

Great verbal and written communication skills are essential as a Senior Designer is expected to present to executives and the overall development team.

SENIOR DESIGNER

DUTIES AND RESPONSIBILITIEs

• Collaborate closely as a member of the Game leadership team to deliver world-class AAA games and experiences that delight users and meet business goals.
• Design all aspects of game systems, from the high-level brief down to the written specification documentation and finally collaborating directly on the iterative creation, tuning and polish of final systems.
• Participate as a product owner of systems, working with engineers and artists on the detailed creation and working with quality assurance and the development managers to assess progress and completeness of features and systems.
• As leader in the design team a Senior Designer inspires and motivates the team to deliver their best work and provides effective and constructive feedback to the design team and others.
• Proficiency in documentation and communication is essential in sharing information and articulating the rationale behind the game direction decisions among the development team.
• Advising and directing the creation and user experience of unique Maxis content creation tools and editors designed for internal teams, outsourcing teams, and customers
• As a master of their craft and one who is experienced in this field, a Senior Designer will help in staff development including recruiting, training, mentorship, and guidance.

REQUIREMENTS

• 5+ years of games industry design experience in shipping AAA titles.
• The unique nature of this product requires prior experience in deeply social multiplayer games such as MMOs.
• Deep understanding of principles of game progression design, crafting trees, technology trees, character RPG-style leveling design and mathematical loot progression modeling (i.e. spreadsheet design skills needed)
• Deep understanding of simulation game design, including simulation loops, wants and needs driven behaviors, and systemic pushback.
• Ability to weave Free-to-Play game mechanics with monetization in simple, natural, respectful ways (i.e. design monetization that players respect and feel adds a great deal of value for their money).
• Demonstrated ability to direct internalize core game pillars and propagate/saturate throughout every aspect of design whenever possible.
• Experience working with teams across multiple locations.
• Ability to effectively direct and delegate to other designers.
• Able to give effective critiques and also take feedback & criticism.
• Solid understanding of popular games, their gameplay, and their art direction.
• Understanding of Maxis creativity and simulation games, including SimCity, The Sims, and Spore.
• Understanding playing and understanding other games in the creativity space, including Little Big Planet, Minecraft and its popular mods, Roblox, Disney Infinity, Project Spark, and other upcoming efforts.
• Passion, energy and enthusiasm for gaming, collaboration, and making things.

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The description of this new game is almost uncharacteristically rich with details, giving us a lot of information on what it might be. To recap, looks like it might be a free to play MMO that mixes the simulation elements that characterize Maxis games with RPG elements and creativity similar to titles like LittleBigPlanet or Project Spark. 

While PC it's mentioned, the wording is a little vague, and it talks about integration, so while Maxis normally works primarily on PC, we can't exclude that it'll land elsewhere as well.

Interestingly enough, Maxis is also working on another unannounced AAA game, but this should be a different one, as that title is in development at the SimCity studio in Emeryville.

The concept definitely seems very interesting, especially considering that "emerging IP" definitely sounds like we're looking at a new one. Unfortunately we'll have to wait for an official announcement from Electronic Arts to know more.