Having it serve to make an allied tower stronger to defend against dives would be amazing, I had a wonderful game yesterday where I was doing very well, until the enemy Shyvanna decided to pitch a tent bot lane, ganked once got the enemy Vayne a double, we could still fight but we were careful not to push too far, next thing I know a dragon is flying at me under my tower and I'm dead in 2 seconds again. At this point my Hecarim support afks, I get dived three more times and we FF the game since the enemy Vayne is now at 7-0.
I would even go as far as making the summoner spell take the laser effect on base towers and turn it into a "shock field," all enemy champions in the radius take damage and deal reduced damage to champions and towers, this effect only functions if there is at least one allied champion within tower range.
Garrison: Empowers targeted tower to deal constant escalating DoT damage to all enemy champions within its effective radius, while reducing their damage to the tower by 50% and damage to allied champions by 15%, this empowerment lasts 10 seconds, once Garrison has been used on a tower, it cannot be Garrisoned again for 120 seconds. Garrisoned towers are spell immune for the duration (no Ohmwrecker, no Bard ulting the tower) CD 210.
I'm against having an offensive use for it since it would make dives easier than they already are, plus I hate taking items out of the game, Ohmwrecker has a good idea behind it can could work really well on someone like Bard since Bard already has a way to "turn off" a tower, with Ohmwrecker he could extend that duration longer