Well, 5E lists 'ignores moral questions' as the only version of True Neutral, so no balancing good & evil option. First thing that comes to mind would be a paladin of a neutral craft/knowledge focused faith. Their code would be something like: to protect knowledge, foster education and training, and be true teachers & inventors. Not sure how well that fits on the basic paladin, though.
Don't see why a knowledge focused 'din couldn't work out. In the Brimstone Angels series, there's a former paladin of Ogma. Some of the stuff from that could work as a basis for an oath. As for abilities, it could be tied to ritual magic or extra skills in some ways. Have to remember to keep it distinct from anything a Knowledge cleric can do - I'm a firm disbeliever in overlap.
Hmmmm...... so.... I guess I'll post some ideas I have below.
People think Vengeance paladins are evil?
Yikes.
That's why alignment is a terrible game idea.
I think that Lawful Evil is actually far less "real world evil" than Chaotic Neutral. But, yes, the Avenger subclass has a strong bounty-hunter/assassin vibe. They're called
dark knights for a reason, after all. The description says that "Paladins who uphold these tenets are willing to sacrifice even their own righteousness to mete out justice upon those who do evil."
One of the curious things about the Avenger is that, when I was reading the writeup, I immediately thought about the Pact Primeval. The ancient pact that Asmodeus made with the gods of light, where he promised to punish the forces of evil in exchange for divine power. The arch-demon pretty much embodies the idea of the LE Avenger.
Now, while we may end up in disagreement over if that should constitute Lawful Evil, I really can't see them as Good as core describes the Alignment. Most PC avengers would likely end up as LN, given that most people shy away from LE, but I see most LE paladins as Avengers before Oathbreaker.
Oath of (the) Ancients: begs the question who the Ancients are - deities or entities of all alignments? A full pantheon? Does it beg a balanced view? Perhaps LN fits best, then TN, followed by LG or LE at the extremes...
Generally speaking, the write up suggests that we're talking about druidic dieties. Also, remember that this subclass is based, in part, on the Primal Defender Warden class from 4e. So "Nature" as a god, a nature goddess, fey gods, primal spirits.
While 3e and earlier put druids/nature as Neutral only stuff, there was always an odd counterpoint with the Outer Planes, where the Beastlands and Arborea were Chaotic Good "idealized nature" realms. Given the discussion of light, beauty, and love in the Warden's oath (and the fact they're called fey knights as well as the prevalence as gods of love/beauty as CG beings) I tend to assuming that this Oath follows the ideals of the outer planes instead of the whole neutral thing.