jgsugden
Legend
If your players are not going to be familiar with the subclass either, I'm not sure there is a benefit to emulating a subclass they do not know. To that end, I'd consider going in a different direction: Make the druid a druid / warlock mix....who is an apostate of two PCs' religion, who believes fostering violence and political unrest will always lead to a better world in the long run. "Burn civilization and collapse institutions and you have made crucible for shaping a better world to build." He mostly focuses on "helping" the oppressed and underclass, but is really exploiting them for his larger vision - convinced that anyone harmed in the process is a necessary sacrifice....
I have no plans of stating him up like a PC - but want to give him some powers that emulate an existing druid subclasses powers. The problem is none of the druid subclasses I have access to feel quite right. ....
If it helps, his allies are a lizardfolk who bears a grudge against the party for previous actions in the campaign and a group of troglodytes the tiefling has promised renewed power after having been driven from their homes a generation ago. Despite the lizardy-ness of the allies, he is not a particularly reptile focused guy.
If they turned on the religions, maybe they've made a pact with another being - something darker. Either an Unseelie, an Archfiend, an Undead or a Great Old One. If you're going to riot and burn down society, you might as well loot and pillage a few lost souls for profit, right? The warlock features make for a great 'distant' enemy (Gaze of Two Minds, Chain-Lock Familiars as spies and voices, etc...) It also allows you to introduce a larger menacing Patron that can go on to be the Big Bad of the next phase of the game.
Personally, I'd go with something like a pact with Dagon (Great Old One) made in the depths. I'd add Armor of Agathys as a spell (which gets nasty when at high level on a druid), Pact of the Chain for a Gazer (with a swim speed) familiar, and a mix of summoning capabilities. There are a number of Warlock spells that make good 'always on' boons (like Shadows of Moil) to give your NPCs a threatening feel. You may also allow them to do things like Wildshape into Aberrations, etc...