Whether directly, like… getting a quest that does something…
Or maybe less direct or obvious, like a hidden mechanic that makes Exp gain you more Favor than normally, or something, since you don’t use that Exp for levels anymore.
No, the only difference between 99 and 100 is that you get one more passive point (as in, you get the one from leveling, not an extra one).
And the xp bar keeps going up after level 100, just nothing happens when you “level up” again.
The reason for this is that the devs don’t want players to feel like they HAVE to reach level 100. For the most part, a level 90 character is pretty much as good as a level 100 one. Especially because the most important nodes for your build are already selected at that point.
When we get achievements we’ll almost certainly get one for it, though.
Hmm. I like the notion of it, which is a lot of Diablo 2’s comfort on the topic as well. I don’t think I ever leveled a char past ~93 in that game.
In Epoch, I think leveling is fast enough to where we could do with some kinda nice benefit, though? (Maybe even exactly what I said; funnel unused Level Exp into other things like Favor, or even just skill points. Or make skills respec as Default Level 20 if you’re character level 100.)
I’m almost level 100 on this char, and I have only been playing for a few weeks!
Don’t let a lot of people hear you say that. There have been countless threads complaining that 90+ takes too long. 
Giving post-100 benefits would just make people feel like they have to reach level 100, which the devs don’t want to happen. You said yourself that it took you a few weeks. That is a long time, especially when many players don’t have as long to play.
It would make it feel like the game is catering only to the people that have time to no-life the game.
And if there would be any benefit to level 100, it definitely wouldn’t be free skill respecs. That would only encourage switching builds to cheese content, which is something else they don’t want to happen.
The devs want level 100 to be something that is a personal achievement that you can go after and feel good about, not something that is required to enjoy the game.
They want a level 90 character to be able to do everything a level 100 one does and not feel like they’re losing something by not chasing that.
Besides, unlike in PoE, getting to 100 isn’t a matter of skill/build. It’s just a matter of time. Since there are no xp penalties on death, you are guaranteed to reach level 100 as long as you keep playing.
Might as well just give a bonus for reaching 100 hours on your character or something.