In its Fiscal Year Fourth Quarter Report (Ending in April 2018), gumi had revealed that gumi Europe is in in the midst of working on a new Brave Frontier game with the working title of Brave Frontier: The Last Summoner, which is to be released for the Android and iOS platforms.
Based on the report, the new game will serve as a spin-off title of the Brave Frontier series rather than a follow-up or sequel. Also, with the game being developed by its Europe subsidiary, the game is expected to be out for the west first.
Unfortunately, little else about the new game is known at this point, although do expect more details down the line. One interesting tidbit is that the game will boast 25 vs. 25 unit battles and is scheduled to be ready sometime in Summer 2018.