I think it would be interesting if they had Shield and Sword Oath change the secondary effects of given abilities, with Sword Oath getting offensive/OT benefits and Shield Oath giving defensive/MT benefits. Examples would include having Sword Oath cause Flash to deal damage (with no bonus enmity) and Shield Oath increasing the static enmity further, Halone getting a DoT with Sword Oath and the 10% STR debuff in Shield Oath, and Circle of Scorn getting the DoT while in Sword Oath but instead providing a retributive damage buff to the tank while in Shield Oath (20 potency per attack for 10 seconds, which, if you were actually being attacked, would provide slightly more damage, assuming you actually maintain threat the entire time).
Of course, I get the feeling that the simplicity of the one-of-multiple stances is intentional. The various Fist abilities that PGL/MNK gets and the PLD Oaths are all simple passive benefits. The only really "complicated" stance is Defiance, and that's only because it defines the entire playstyle of WAR: the stance itself is simple (increase hp, reduce damage, and generate Wrath stacks with attacks); it's the interaction with the other WAR abilities that makes it complicated. On top of that, the only "stance dancing" that ever really goes on is healers dancing in and out of Cleric Stance for short periods (I tend to drop into Cleric Stance just long enough to apply and reapply DoTs if I have to heal regularly or spend most of my time in Cleric Stance to DPS and just drop out just long enough to refresh HoTs/shields or throw out an AoE heal) and I'm still not really sure whether *that* is intentional: Cleric Stance was obviously intended to be a "soloing" stance (completely gib heals to give you straight up DPS capability) so I'd have to wonder if they figured that players would use it to boost damage during a couple open GCDs before turning it back off (mostly because it's a free stance that's off-GCD, unlike the PLD stances and Defiance).