I always thought the nexus turrets should be combo turrets, a mix of inner and outter turrets. They shoot a beam like inner turrets, but also an energy blast like outter turrets, but only when inhibs are down. They also cant energy blast the a beamed target, but the adjacent one could. This would mean the nexus turrets could take on up to 4 targets.
It would be neat to have a summoner spell that gave a turret a damage aura, nothing huge, but enough. Say starting at 50 DPS true damage, escalating on a target by 10% per second. Cooldown could be 3 min, with a 10 second duration. Could need tuning obviously. This wouldnt turn away a late game triple dive scenerio, or Kat/Zed dive, but it could turn away someone whos planning on tanking 2 hits to kill u then flash out.
I like the dual target beam/blast hybrid idea, sure its mine, but I think it would be the most interesting than a strait AoE blast.