Storm Bulwark in the normal game mode

Janna's shield alone isn't enough to defend a tower, but there is no other option on any other champion or item to defend the tower itself aside from fighting the enemy.
It would be reasonable, though, to allow the support to give up Exhaust or Flash to have a weakened version of Storm Bulwark in the normal game mode.

I won't suggest something insane like bringing back Fortify, but the Bulwark seems like a great option for defense against multiple-enemy ganks (particularly at bot).