First of all I was very sceptic to the Dungeon idea, making a forced multiplayer mode, but it turns into a really nice team work and fun we (could*) have around it!
*yes... we could. Sadly the RNG based rewards, annoying need for a repeatability and kind of pointless grinding X-mass coins killed it after very few attempts taken. I can understand that designing such a complex quest is timetaking for staff, and you would like to be rewarded with high playtime we spend on *enjoying* it. Sadly, how it looks you must be more likely satisfied by watching how hardly players can humiliate themselves by trying hard to complete the new content rather then enjoy the storyline.
How could to improve quests IMO:
1. Announce a clear rewardpool/rules/mechanic of scoring at the very beginning of new additions.
2. Minimize RNG factor. Seriously. Hax are the part of the game. The annoying one. Make rather the encounters harder to beat than randomize the rewards for completing (Thomas, the stone, ...)
3. Prepare some better rewards for such things as X-mass coins, or set up a converter to PvE coins. Grinding on multiple dungeons runs while completing the stone and Thomas earlier, would at least benefit at some extra coins to spend for Masterball, Rerolls, mounts, train ticket, etc. Valuable (even untradeable) items.
4. Once making a random party, an automatically created in game chat-tab would be a necessary addition.
5. Newley rereleased legendries should appear in battle in event form, but be catchable in regular form. For collectors, getting the extra from should be a part of some extra actions taken, not the opposite.