Increase EXP from side quests once main quest complete or max lvl a class

[Seraphor]

And they give almost no exp. You make more killing a few things in the same time. It really is very lopsided.

Not true at all.

You might make more from the time investment by doing a FATE, but just killing random mobs, absolutely not.
And they don't want side quests to be more lucrative than FATEs, because then no one would do FATEs, they'd just do the sidequests where they can skip five pages of text in 30 seconds and instantly get 20% of a level.

Now maybe they could add a challenge log prize, a daily bonus for doing say, 1, 3 and 5 side quests, which are comparable EXP to doing 1, 2 and 3 FATEs respectively. That could work.