In our modern world, morality has become very subjective because everyone disagrees with how to define "good". Good has become subjective. But in the BG world, deities actually exist and have their own views of what is "good". They give the paladin power as long as the paladin adheres to that definition.
Well,
Euthyphro’s dilemma appeared in the Platonic dialogues, long predating any modern moral relativism, and was originally raised in the context of a polytheistic pantheon somewhat like FR’s, so I think there are some definite question marks over your moral philosophy there, but I’ll not take us further off topic into that arena!
I do agree with the key point I think you’re getting at, though, ie that the existing oaths we have (Devotion and Ancients) both have tenets that align closely with good (in the FR sense), so the only way to play an evil paladin is to be an oathbreaker, whereas it should be possible to play a paladin who makes, and keeps, their oaths to an evil god.
Given I don’t believe it’s the place of BG3 to overhaul the 5e approach to paladins, for me that means introducing an oath or oaths whose tenets are more compatible with an evil god, and I assume we’re going to get Oath of Vengeance (as it’s PHB and I think Minthara already has some of its abilities) which would at least give us one option there.
I actually don’t want the game to warn me if I’m about to break my oath, though, as I’d feel that would cheapen and gamify my paladin’s decision making. But I would like (a) the basic tenets of each oath to be clear in the game, (b) oathbreaking to make sense in light of those tenets, (c) the ability somehow to argue
after the event if I don’t agree that the action broke my oath.
With respect to deities, I definitely agree that we should (optionally) be able to specify a deity for paladins, and it would be great if the game could respond to that on top of responding to the basic tenets of the oaths we select. But I do recognise that there will be practical limitations on how far that could be done, given the number of deities.
