I am considering to maybe give 5th edition another chance in my next campaign (since people often say about my complaints that you don't need to run the game the way I did), and want to come back to this idea.
In the vague setting idea I have in mind, there's certainly room for barbarians, fighters, monks, and rogues, excluding the spellcasting variants of the later three. And of course warlocks.
But I've been thinking about ways to broaden the options for PCs some more, and I'm now wondering about the possibility of revamping the bard as a kind of half-caster warlock. The college of lore is already thematically about scholars of the arcane, and in a world without wizards, where warlocks take on a role more like cultists or priests, that could be an interesting aspect to focus on.
Druids could also be an interesting counterpoint to warlocks. With the circles of the land, the druids are implied to get their power directly from the minor spirits of the environment. This could be contrasted to warlocks having connections to powers from beyond. The only downside I see with that is that druids might outshine the warlocks as the main wielders of spells. However, if making long rests less easily and frequently accessible, the warlocks' ability to regain all their power at simply a short rest might rebalance that. Perhaps add more invocations to add spells that don't use slots and are effectively unlimited for warlocks.
(Not sure how much the typical Longer Rest variant would impact things, considering I am expecting fewer fights, but more potential uses for non-combat spells.)
My idea with that is that druids would really only be found as barbarian shamans, and as such perhaps come across as even more strange in civilized areas where people are somewhat used to warlock-priests using their kind of spells.