I've made a Paladin that generally focuses on protecting with shields and bubbles. Think world of warcraft I guess. I'm a triton who was resurrected by a sun god. So anything light-themed or water-themed is usually my go-to. I really enjoy the idea of persistent spells too. EG Devotions Lv15 ability. My current placeholder is "minus damage based on CHA mod." but worded better. I'm just not really feeling it and would enjoy some community input on a Level 7 aura. Maybe even using the Level 15 as an improvement to that aura too.

Oath of the Avenger Spells

Devotion Aura

By 7th level, The mere presence of a powerful paladin can instill great courage and inner strength in those around them. This powerful, spiritual surge actually increases the defensive capabilities of those gathered near the paladin. Whenever you or a friendly creature within 10ft. is hit by an attack, its damage is reduced by an amount equal to your Charisma modifier.

At 18th level the range of this aura increases to 30ft.

Well first If thats your idea for your Paladin class why do you have the shield spell? It cant be cast on another creature just yourself. 

I dont see an issue with this. It fits your intent however Its usefulness compared to other Paladin Auras has serious drop off at higher lvls.

The shield spell was to replicate the WoW paladin self bubble. I'm not making an exact copy of the WoW pally, but it's got some fun features.

Thats understandable then. I've never played WoW but if your going for the thematics of it it makes sense to me.

What if, to balance out the aura but make it equal in power to other auras at high levels, the damage reduction is half your paladin level?

Or if you would rather have level 15 improve it, add your prof to the damage reduction.