1. Unless the tanks are far beyond each other in capability or gear, there shouldn't be an issue with threat with proper management.
2. It's going to happen anyways. Getting rid of the threat bonus will not stop tanks from tanking in DPS stance, because as of right now, more deeps is king.
3. The current trend is that SE is moving away from the class/job armory system. Notice how none of the classes added past 2.0 provide any meaningful utility to the other classes while they all just essentially draw minor (but required) abilities? It's no coincidence that Machinist draws from Archer/Lancer, or that ninja draws from Monk/Lancer; They have abilities that are near required to remain competitive on dps. I wouldn't be surprised if in the future they fold these abilities into every base class that needs them.
4. Eh, it's a fair point. No reason all stances shouldn't be on OGCD.