I've brought this up earlier, but it's been years so I figured I'd try again to see if people agree.
I love to level new alts every once in a while, and the scaling tech being introduced to the old world in BfA got me eager to do it again. Without heirlooms it goes a bit too slow, so I always use them because of the exp-bonus. But the heirlooms also removes some of the most fun parts of leveling; the feeling of getting a nice upgrade that makes you feel stronger. Mobs dies in a few hits anyways, but just vendoring an item you just got because it goes in a heirloom slot feels a bit lame.
I would love for the exp-bonus from your heirlooms to be a buff in the form of an item like the anniversary-gift. If you only have the head and shoulder heirlooms, you'd get a buff item worth 20% exp bonus, but those two slots would be free to use for quest-rewards or drops. This would probably make your char a tiny bit weaker, but in my opinion it would make leveling more enjoyable.
What do you think? Would you still use heirlooms, or would you just apply the buff?