People have been asking for this for quite a long time already, and Wizards of the Coast has finally decided to grant their request. On 14 June 2019, Modern Horizons will be released and 249 new-to-Modern cards with it. That's right, every card in the set, save the basic lands, will be brand-new and Modern-legal! However, they will notably not be Standard-legal, signaling a shift in how Wizards will be treating Modern. So far, only a mere two cards have been spoiled: One is a reusable Cabal Therapy on legs; the other is a Serra planeswalker, so think Serra Angel with two other sweet effects.
Check back in mid-May 2019 for the full expansion breakdown, where we will see just how much this set will shake up the already varied Modern format.