Theoretically Gunblade could be a DPS Job, but that means we'd be seeing three Jobs released with an expansion again, and I do believe the Developers stated that was very difficult with Heavensward, especially when they had implemented Ninja just prior to it. Implementing Blue Mage just prior to Shadowbringers would be repeating the issue if we were getting three new jobs.

Right now we really need a new Tank and Healer Job each. Players of both roles were extremely upset with Stormblood only providing two new DPS jobs.