Do you allow masterwork weapons and armour?
A weapon only gives a +1 to either attack or damage not both, whilst armour merely removes the disadvantage effect as if it was mithral or titanite as long as its medium or heavy type armour?
How are you going to handle the heirloom part?
For example say one character has a moonblade that when sheathed has the form and size of a dagger, but when drawn extends to either size of a shot sword or long sword its easier to carry.
Would that count as a suitable heirloom magical item despite not being on that Table B?
Knowing your players will they come up with their own ideas for a heirloom item?
I hadn't planned on allowing masterwork items. I think the "mythic materials" part serves that purpose already.
The heirloom part is pretty straightforward: the player picks a magic item from Table B, and then invents a reason for why it's so important to their character. It might be a family heirloom, but it might also be the last gift from a departed loved one, a keepsake from a terrible foe you slew, an item that was dropped by the man who murdered your mother, etc. As long as it is important to your character in some way.
I don't mind if the players customize them somewhat, too, as long as the changes are minor and the base item is from Table B of the DMG (only). Like, back in the day, I played an Illusionist with the Entertainer background. His Table B magic item was a
bag of holding, but it was reskinned to look like a top hat. Whenever he needed something, he would "pull it out of his hat."