I think the only point that I agree with NastyCloud on is that it's a shame I will forever have empty spots in my Monster List never to be filled.
(Background: Started playing after Wonderland began. I'm level 113 now. Missed out on everything except the pinnos in light and dark first time around. Only have Dorothy and Karen from Insane, can't do Hell. Got Hayyel and my best cards were Warrior Princess and Zeus. Missed out on Azrael. Got a Panzer with Sailor Miles to get Anauel, don't use it for much else except Holy Seer at the moment.)
Now, it would be nice for all players, new and old, to at least have a chance to complete the list, but I think rotating back old time-limited bonus quests, guild war bosses, and raid bosses is a horrible idea. Personally, I would much rather the devs focus their time on getting us new material faster.
With that being said, I do have an idea that MIGHT make the MOST people happy:
Once a year, let's say around the time of the game's anniversary, we get a single weekend (in order to make players choose between the mines and these dungeons) to try and get old cards. (My example will only be for time-limited quests, no guild war or raid bosses or sailor mile gods). There will be a single level for each card (i.e. Dark Panzer, War, Dorothy, etc.) and the chest drops will be the same as for insane (two trash cards in brown, a trash card in gold, and the feeder card in gold) with the bonus being the desired 7* card. However, the level will have the difficulty and drop rate of the hell maps.
New players (or seasoned players I should say, because new players will have a ways to go before they can finish the map) get the opportunity to fill the monster list and have a 7* card from older dungeons.
Veteran players will have the opportunity to pick up one they missed out on or get a second one to evolve to their 8* counterpart (except panzers).
More veteran players will still have the benefit of having been able to farm the cards when the original map was out.
It's a once a year thing for 2 days. For most players if they are lucky they will be able to get 1 or 2 of the cards (w/ wonderland when it's over that's 15 total cards). The luckier/stronger players might be able to get 5 or 6.
People can no longer complain that don't have the opportunity and if you're not willing to be patient then I don't think you truly have an respect for the devs or the integrity of the game. If you're not strong enough to get one of these cards you most likely are strong enough to build a deck around one either.
I would much prefer this as an anniversary present from the devs over a bonus gem sale personally. Just thoughts. I'm curious to see what the rest of the community thinks and if they have any feedback....devs