The last 7th Dragon game is the 7th Dragon III Code: VFD which released in 2015 (Japan) and 2016 (North America and Europe) on Nintendo 3DS.
Well, SEGA about an hour ago posted a strange message on the Twitter with the official account of the game. It looks like SEGA is working on something with the game and they release the fresh news soon. Some people already asked SEGA, is it a Nintendo Switch port version or a new chapter? Well, we will find it out soon.
— セブンスドラゴン公式アカウント (@7thD_Official) July 3, 2017