I think that if you want to build a 'Gunlance' job, it probably should reference Nael's moveset with her gunhalberd Bradamante. You could have a Ravensbeak combo that has a Wildfire type effect, an Iron Chariot/Lunar Dynamo AoE combo, and Ravensdives as gap closers. Players have been asking for a Legatus/Judge type job for a while, and that's one way of doing it. The difficulty is in making it visually distinct.

If I had to guess, though, I think we're sooner likely to see a hammer/maul based tank first. An Engineer type with Nero's rocket hammer is another way to do a Garlean themed tank.