Ohmwrecker could use some love, and there are plenty of ways they could add defensive functionality.
Passive:
1: Apply a growing shield to nearby towers. If you remember the shield that the outer tier 2 turrets gave to champs before, I'm suggesting something similar but in reverse. Standing near a turret would continue to place a growing shield on it up to a cap and the shield would fade if you left it's range.
Active:
2: Make an ally turret shoot faster. This is extra punishing to people who dive and help turrets clear minion waves faster.
3: Make a turret's shots deal splash damage. This doesn't punish divers more, but the waveclear is still enhanced and all enemy champs that want to avoid damage would need to stand away from their minions.
4: Perhaps Riot would be willing to bring back a reworked Fortify through this item. You would need to be in range of the one turret you want to buff (unlike Fortify's global & all-turret buff). Fortify helped with divers and waveclear by increasing the turret's attack speed on top of making it invulnerable for a period.
- An alternate take on this could have it work like Tryndamere's ult, the turret would be able to take damage until it hit 1 hp. If this buff persisted for, let's say 15 seconds, then it would prevent those dives that happen right before a turret falls and would still be worth considering even if ranged champs would be able to auto the turret once after the buff wears off. Much more conditional that the normal version.
5: Give a turret the ultimate anti-minion buff. This could only be used on a turret once a nearby minion applies the armor-reduction to the turret and this buff would persist until the armor-reduction would go away. Enemy minions will deal no damage to the turret and all shots from the turret will instantly kill any minion hit. If an enemy champ would draw aggro, the turret would receive an offensive buff (speed/damage/range/etc.) until it went back to using it's standard buff to kill the minions. This would help during conditional dives and would help against mid-late (especially baron enhanced) pushes.
6: Reduce all damage the turret takes for a period. This doesn't punish divers, but can extend the lives of the turrets since their armor was nerfed this season. Not as useful during laning phase, but it could help those defending against the mid-late pushes. It wouldn't make turrets invincible, but the added defenses would deter pushes where the defense has at least the same number of champs as the offense.
7: Force the turret to shoot an enemy champ in range. This could work by either selecting the champ you want it to shoot or by having the item automatically select the enemy champ with the lowest current health. This could be used to get enemy champs away from your turret, change the target during a dive from their tank to their carry, or finish off a low health enemy that's in range of your turret.
- An alternate take on this could have the turret lock onto all enemy champs in range until they each leave it's range. This would heavily punish dives but not help waveclear at all.
So plenty of possibilities are there, just have to find one that would feel rewarding, fair to play against, and not become too strong of a choice. Of course those could also come down to the numbers on the buffs, but with that item's current niche it needs something to be worth picking over the other items.