Benjamin Olson
Hero
I guess you could if you want your game to be a clash of messy elements with no design care. It does not seem advisable.You in every case can, thus far.
Looking just at the most recent playtest, using the new moon druid subclass with the old druid, or vice versa, would involve meshing two incompatible systems of wildshaping, since the subclass revolves around that. And while moving the level at which features arrive by a level or two rarely matters much, so shifting around a little as a homebrew fix is not the end of the world, Paladins are shifting to getting their 3rd and 4th subclass features at 15 and 20 to getting them at 10 and 14. While the example we've seen of the Devotion Paladin is perhaps not too radically affected, it was still tweaked a bit to account for the level 20 ability being demoted from class capstone to something you could have from level 14 on, as well as replacing a level 15 ability with a level 10 ability.
It's all doable, but I don't see subclasses from one system as out-of-the-box, readily compatible with the other system, and it's kind of an unreasonable level of compatibility to ask for when the new system is standardizing subclass progression and 5e went in a very different direction. In any case they clearly are not building around the idea of intercompatible subclasses. I feel like the standardized subclass leveling was introduced in part just to signal to people not to expect that level of intercompatibility.
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