The issue is that there isn’t any way to give Varian a stun that isn’t tied to some form of point-and-click. Even if it was a skillshot, you can still get into range with Charge and then force someone to stand still with Taunt, making the skillshot effectively a point-and-click by extension because… well, they’re standing still.
The other tanks are tolerable because their stuns are skillshots (with the exception being Diablo’s Overpower, which is incredibly short-range) and the player has the opportunity to dodge them. Even in the case of chained stuns, the player has to get hit by one first.
Smash and Twin Blades’ periodic short stuns weren’t the issue (although they were strong), the issue was the total lockdown that Taunt and Warbringer gave when combined. Because of Taunt itself, there’s almost no way to add in any additional hard CC without making Tank Varian completely insufferable again.
As for Talents that alter based on the previous build choices you’ve made, I’m actually a huge fan of that concept and designed my entire Xul rework around it.
They wouldn’t stack, but the banner wouldn’t become useless by any stretch of the imagination. Intercept grants Physical Armor, whereas the Banner of Ironforge grants Standard Armor. Combining the two would basically be giving players 50 Physical Armor and 20 Spell Armor while benefiting from Intercept and 20 Armor otherwise.
The point of Intercept is to provide armor to your allies, not Varian, so that wouldn’t fill the role very well.
As I said above, it’s very difficult to make that happen in an organic way that doesn’t tie into a point-and-click mechanic somehow. The best I could offer is maybe extending the duration of Taunt itself, but that’s pretty dangerous.
Not many heroes require that, no, but that’s how fulfilling a niche goes. Varian is one of the few that can fill that niche, but it comes at the cost of being able to do other jobs as effectively as his peers.