A seasonal reset would already do this, but I am thinking specifically for within the season.
Rested Currency
Allow players to accumulate rested currency that they can spend upon completion of a dungeon, allowing for multiplication of the rewards.
For example,
You gain 1 coin/hour offline. Capping at 2 weeks, 336 coins. (can adjust this number)
At the end of a dungeon, get an additional payout of loot for 8 coins.
You could limit it to that or allow them to spend like 24 coins to get multiple payouts.
Capstone could either be ineligible, halved or even full (it's something you can min max, and skip the grind of spamming capstones before full-sending onto the next tier).
This is less efficient than just non-stop running dungeons, but it allows more casual players to catch up with their friends, and overall make the game less grindy. It will also give a pseudo first-dungeon-of-the-day rewards to regular/hardcore players.