In my homebrew mashup I use a lot of Golarion aspects so the Kellid ethnicity is a common one in many areas and they are a fantasy Celt/Cimmerian ethnicity.
I often have strains of druidism being fairly strong among the Kellids.
So in my Holy Lothian Empire imperial province of Ustalav where Kellids are a rustic conquered underclass druidism is an older indigenous faith that the imperial henotheistic state church at times has tolerated and persecuted, associating it at times with rustic superstitions, demons, old ones, and witches.
For my Ustalavan Kellids druidic faith tradition mostly comes down to a Wheel of Time type of belief in reincarnation, a respect for druids as advisors and people of power, a bit of reverence for nature and fey and maybe some specific cultural gods, and some seasonal holiday traditions. They might be good Lothian's who also keep older traditions alive alongside the empire's ascended paladin god. When I ran my 5e Carrion Crown adventure path game in Ustalav druidism was mostly a background setting element fitting into prejudices and divides that were themes in the gothic horror game.
For my Numenerian Kellid tribal lands outside of the Empire's borders where my current 5e adventure path campaign is set the Kellids are the majority and Druidism is a strong cultural factor. In this part of my setting I had the Werewolf druid character so Druidism has a strong White Wolf Werewolf the Apocalypse kinfolk aspect to it. Spirit Realms, Totem spirits, spirit challenges and pacts, the fight against the Wyrm, distrust of the Weaver, werewolves patroning Kellids and driving some of the beliefs. A lot of the White Wolf stuff fits in easily in a D&D context and makes druidism have some hooks to roleplay out and riff off of.