You know, i belileve, one of blizzards most devastating decisions ever was to devalue old content, by making old crafting useless, by making old gear useless, by making leveling rewards as like gear useless. By making the whole old world obsolete.
One idea to make old content mattering more would be to make leveling and leveling rewards actually useful again. Which includes character progression with gear upgrades.
That would mean, heirlooms, as they are now, should be replaced or removed. As blizzard surely would not like to give up their heirloom tab, and the hundred thousands of gold they sell them for (tokens!), i would suggest to transform heirlooms into leveling enchants to items, which give additional experience, and some minor stats upgrades. Instead of a chestpiece which levels with you, you get an enchant for your chestpiece which raises int or str or agi by 2%, and your experience by 5%.
Same goes to the other heirlooms. This would mean, that leveling would return to character progression. The heirloom tab still would be there, but once you got a new item, you had to reattach the heirloom enchant to the new item.
Just one of a possible hundred ideas to make low level content more compelling. Another idea would be to add difficulty levels for questing. You had easy, normal and heroic questing. Easy would be what we had prior to legion, Normal would be after legion leveling difficulty, and Heroic would be a difficult level of questing best played in a group. The higher difficulty would reward more gold and vanity items.
What would be your ideas to make old content more compelling?