If could add anything with no restrictions to the game.

[Fyrebrand]

The xp you get from MSQ is certifiably insane. Reduce the xp you get from MSQ by 25% or so, but have side quests give ten times as much as they do now. Playing through an MMO where all the side quests are obsolete trash content to be rushed past and ignored makes the overall experience feel cheap and sad. It just sends the message that every zone you're in doesn't matter, it should be skipped, it's old bad content, don't even look at it. Who wants to play a game that feels like that!?

Some of the side quests are pretty neat! They have cool story bits, interesting lore about the world, or else just make you feel more connected to the place you're in. It makes the game more immersive, and lets you feel like you actively EARNED your experience rather than were just given it by some fast-track progression path. Plus, it would make leveling alt jobs smoother.

Bump up the xp from Fates as well.

Also: when a player is still progressing MSQ, and they've reached the level cap for whatever content they're in (50 for ARR, 60 for Heavensward, etc.) lock their level progression and don't let them over-level the content. There's no need for it, and it leads to some ridiculous situations like players getting locked out of roulettes or else being way too powerful for solo and open world content. For ARR and Heavensward at least, they don't even get access to new spells and skills anyway because they can't do their job quests. There is no benefit. I was level 90 before I was done Shadowbringers. It was stupid, and it made me not want to play my main job.