[Update]: Khans of Tarkir and the Timeless format have now arrived, though not without some server trouble!
In an announcement that shouldn't come as too much of a surprise given the recent experimentation with Historic 'no bans' events, Wizards of the Coast has now unveiled a brand new format for MTG Arena - Timeless. As the name might suggest, Timeless will be a 60-card, ridiculously high powered format encompassing every single card ever released onto Arena, and no bans whatsoever!
That said, the last part is only technically correct. While no card has been banned, three of them have been restricted, which means you can only run one copy of them in your deck (sideboard included). The deadly trio is Channel, Demonic Tutor and Tibalt's Trickery, with the latter being restricted purely to prevent stupid non-games where the victor is decided based on a dice roll on turn 2.
Whenever possible, Timeless will use the original tabletop printing of all non-digital cards, so powerhouses like Teferi, Time Raveler or Minsc & Boo, Timeless Heroes will be fully unleashed. As for the digital-only cards, those will use their rebalanced form if any is available.
If this sounds as exciting to you as it does to me you'll be happy to hear that Timeless will be launching next week, on December 12th, alongside the release of Khans of Tarkir! Since this is going to be a competitive format you can expect the usual suite of ranked and non-ranked game modes, as well as a variety of promotions and events to help introduce players into the crazy world of high powered Magic.
The introduction of Timeless as a format doesn't mean that Historic is going away, however! According to Wizards of the Coast, the plan for Historic is to remain a highly curated format with a high yet reasonable power level. As such, the Khans of Tarkir fetch lands have been pre-banned from Historic due to how much they would warp the format, though you'll still be able to use them in both Timeless and Historic Brawl (which looks like it'll be going through a name change in the future).
Once Timeless actually arrives and we all get to bash each other silly on turn 2, I'll make sure to let you know. Until then, I'd recommend checking out the developer blog detailing Wizards of the Coast's plans for the Timeless format and its future. Enjoy!