I'd prefer it be whatever the druid uses. Ranger's spells/nature powers are like a skilled learned out of necessity. Wild animals and poisonous plants everywhere, gotta be able to charm/hide from/detect the beasts and remove the poison.
Some of my preference comes from how I've defined arcane, divine, and psionic magics since 1E:
- Arcane is power drawn from the cosmos, whether that means ley lines, ambient energies, the "heat" generated by planes rubbing together, or something else is secondary. There is some rational system by which the power can be harnessed and put to use. That's why arcanists rely on knowledge (whether gained through study or revealed by pacts with other entities). It's all learned.
- Divine is power that comes from another being that has the ability to revoke the power, at will. Low level divine spells may gained from the "penumbra" around a being that is worshiped by the cleric, but even those could be denied or transformed if the deity should so choose. By the third spell level, the spells are granted by an agent of the divinity. This lines up with Gygax's commentary in the 1e DMG. Divine magic doesn't require knowledge, so much as commitment to (faith in) a power capable of granting power (and willing to do so).
- Psionic powers come from within the "caster". There is something different that lets the psion generate power that mortals should not be able to. This could be some act of faith, god blood, or something else. In my campaign, it generally comes from contact with some artifact or relic, usually by way of an ancestor. There was also an empire that had performed so many arcane experiments on it's populace and lands that there was a very high rate of psionic "mutation". Psions are the "born mages".
By those definitions (i.e. my bias), the ranger cannot be a divine caster unless the base purpose of the class is to serve a deity. Additionally, if the magic is based on learning, study, and general resourcefulness, it must be arcane. I always took the 1e ranger spells from the druid list as an expedient way to say the ranger learned a very odd form of magic, nothing more, and was a bit annoyed when the wizard spells, rather than druid spells, were stripped away in 2e.
For obvious reasons, the idea of clerics of a philosophy or force are utterly absurd, to me. They would be anathema to what a cleric is. It is simply not possible to be a godless cleric -- the binders from 3.5
Tome of Magic would be about as close as you could come.
Then a druid takes on the arcane/divine slot! It may be named differently, but it could stick with that flavor and be a heck of a lot like the early D&D druids.
Druids are an odd case. They are named as divine casters, but the implication is that they may not have a deity, and revere nature instead. The easy answer is to make them animists -- the nature spirits work in very similar ways to how angels and other divine servants work for clerics, but without the "big cheese" pulling the strings. Instead of a deity, you have the overwhelming mass of nature spirits -- from the minor cat, dog, or plant ephemera to the nymphs, dryads, and their ilk to the mightiest of outer planar nature spirits. Those planar spirits might have to join together to provide upper-level spells, but it would be worth it to aid one of the handful of truly mighty champions of nature (or to further the alien goals of those spirits -- read the Werewolf book, sometime).
Or, for less thought, just make the druids be priests of nature gods. It just depends on how much flavor you want backing the mechanics. Likewise, it's possible to frame clerics as simply arcanists using "white magic", which is what TSR did with the Lankhmar setting books.
