Will Batman Beyond be in LEGO Batman: Legacy of the Dark Knight?
Yes — as a DLC suit pack. The Batman Beyond Pack ships with the Deluxe Edition and adds a Batman Beyond suit for each of the 7 heroes.
Batman Beyond is included via the Batman Beyond DLC Pack, which adds one Beyond-themed suit per playable character (e.g. Batman's classic Beyond suit, a Beyond-styled Robin, Nightwing, Batgirl, Catwoman, Gordon and Talia). The pack is bundled with the Deluxe Edition or sold separately. Batman Beyond is a suit/skin only — there's no playable Terry McGinnis as a separate character in the base roster.