The idea of a non-standard supplemental set for the modern format was floating around for a few years now. That idea is now a reality. Wizards has just announced Modern Horizon and the 1st cards spoiled are exciting.
The Modern Horizons set will have 249 new-to-Modern cards (254 total). There will be a miz of reprints not yet legal in Modern and also some new cards. The Pre-Release will be on June 8th and 9th.