I quite like this idea.
I’ve levelled a lot of alts, more than any semi-sane person should and I do use heirlooms to do it. That %XP boost is nice.
But I do miss the feeling you get when upgrading a piece of gear, especially when you move into a new expansion area. I remember with every expansion (except BFA funnily), picking up a random loot drop or getting a piece of quest gear or even making a piece of amour, a shield or a weapon and the joy at watching my stats climb with the new gear.
I don’t know about others, but now with heirlooms, just about all of the gear I pick up while levelling is sold off, unless I’m levelling an enchanter. Then I use it to level each expansions enchanting. Even if it has slightly higher stats than the heirloom it would replace. I know the heirloom stats will surpass it soon, so I don’t bother, I keep the XP boost and take the gold.
This is not a “kill heirlooms” post, I like my heirlooms, the XP boost is great for alt levelling.
If I might suggest a small addition though?
Start the consolidated XP high and have it drop off as you hit higher character levels, until it disappears at say 115. That way people get a good boost early on and with things slowing down, hopefully enjoy playing instead of purely powering to max level.
It’s ridiculous how quick you can level now, even without using all available heirlooms. I started WoD yesterday on an alt with a few heirlooms, after a few sizable breaks from gaming (hours) and doing hardly anything it’s up to level 93 and in Shadowmoon Valley, with Velen still in one piece.