The single biggest issue holding back R Druids is their lack of powerful damage mitigation for their team. The meta is currently so burst heavy that you need multiple powerful damage mitigation effects on your team if you want to be competitive.
Ironbark is currently insufficient as a damage mitigation cooldown, especially when you compare it to something like Blessing of Sacrifice.
The relative weakness of Ironbark forces Druids to gamble with their globals when offensive CDs are used. Ironbark should be a primary reaction, but it often needs to be comboed with either Nature’s Swiftness → Regrowth or Cenarion Ward → Adaptive Swarm, both of which are often too little on their own to deal with something like Combustion or Avenging Wrath.
The best solution would be to make Ironbark grant more damage reduction (bringing it closer in strength to previous expansions), but doing so without any tradeoffs would make Druids potentially overpowered, especially with upcoming buffs.
I think the most OP part of Druid right now is their survivability, especially with Guardian Affinity. The autoproc Frenzied Regeneration (https://www.wowhead.com/spell=340553/well-honed-instincts?ilvl=145) combined with Innate Resolve (https://www.wowhead.com/spell=340543/innate-resolve?ilvl=145) and Ooz’s Frictionless Coating makes it nearly impossible to kill Resto Druids outside of the most coordinated setups.
THE FIX
To buff Ironbark while offering a trade off, I believe that there should be an additional Endurance Conduit that gives Ironbark more damage reduction (scaling from 16% to to 24%).
It would be important that this would be an Endurance Conduit because it would force Druids to trade off their already powerful self-survivability for more team defensive options. The Endurance Conduits available to Druids are far too powerful defensively so this trade off would give them more comp options and an overall more rounded defensive toolkit.