Why 2015? Considering the past games, since Gothic 2: The Night of the Raven, the German company has released one game every three years.
They have released Gothic 2: The Night of the Raven in 2003, Gothic 3 in 2006, Risen in 2009 and Risen 2: Dark Waters in 2012.
Another important reason to consider Gothic 5 as their next game is because they got back the licence for the “Gothic” brand after they have released the second Risen game.
Gothic 3 was the last Gothic game released by Piranha Bytes. The addon that followed Gothic 3, Gothic 3: Forsaken Gods was developed by Trine Games.
Arcania: Gothic 4 was developed by Spellbound Entertainment and published by Jowood. Jowood is the company that owned the Gothic licence until Piranha Bytes has released Risen 2: Dark Waters.
There is nothing certain about a possible Gothic 5 but it is very possible that Piranha Bytes is already working on the title.
You can follow Piranha Bytes on their official website, pluto13.de. And on Facebook. This is their official page. Update: the link was removed, the Facebook page is a fake account.