Why is Druids limited to RoK ?
that seems to be a flawed assumption.
Druids goes for neutral civs. (in the same way eidolon go for evil civ and paladins to good civs)
you can be neutral with different ways :
-originally good : go fellowship or OO
-originally evil : go fellowship or RoK
-originally neutral : stay neutral.
Once you are neutral you can stay neutral and still change into any of those religion : FoL /RoK /OO /Cult of Esus.
you can thus still build druids while being in any of those religions.
Order/empy moves anybody to "good"
Ashen veil moves anybody to "evil"
FoL moves anyone to "neutral"
OO moves good to neutral, neutral stays neutral and evil stays evil
RoK moves evil to neutral, neutral stays neutral and good stays good
CoE doesn't change alignement. (or does it makes you evil ? as anybody else see you as still having the former religion)
a nice strategy is :
convert /build the unit you want to be druid (priests or special harlequin/ranger) change to neutral through religion shuffling, build 4 druids.
next turn change to the religion you want with the alignement you want.
(even better : profite from being with those non-neutral alignement to build 4eilodon and/or 4 paladin)
in the end, you'll get 4druids, 4 eilodon and 4 pallies.
For someone evil wanting OO nice strat :
-go OO (still evil), build Hemah, build some priests...research techs : (eilodon / commune with nature)
-go Order, becomes good, lose Hemah, build some confessor, and pallies. (some 5-10turns). (hope for no other OO follower)
-go OO, becomes neutral.
1) build Hemah once again either you'll still get the former promo or he become a virgin Hemah you can build more appropriatly. as you are later in game you have more mana sources and thus can get much more free mana promotion into Hemah than earlier.
2) get some of those cultist into druids to have krakens (1-2)
3) get all other druid posts be taken by confessors : upgrade to combat V and command IV. (very nice)
even if you change alignement (good/neutral/evil) the already built pallies/eilodon/druids will still be there. but you can only build/replace those your alignement allows for.