I got this idea which could make going through old levels more fun - Treasure hunt!
Somewhere, a mission would pop out (like the special offers would be best mechanism - maybe Vivica would inform you that she found something interesting).
This mission would start with a riddle of some kind pointing towards a certain level (not too hard a riddle, people would share the answer anyway). After completing the level, another clue would appear. This would go on for about 5 levels until you reach a treasure chest containing some goodies. You have to go through each level to receive the chest from the final one. You can’t just skip to it.
There would be several difficulties which would vary so that all players would have a chance to find the treasure, but only players who finished certain levels would be able to get the better treasures.
Making the riddles unique could prove difficult, so maybe just the last one could be a riddle you have to solve while the previous stages would be solved by your companions. This way each player’s path could be different (different level of the same stage), only the final place would be the same or generated from a limited pool of options, because once used, the riddles would have a solution online.
An option would be that completing the final level would spawn a special quest for a 4* ascension item, so that the chain wouldn’t be finished just through loot tickets.
I want this to also be too difficult for lower level players, so that the older players could get something nice.
The timing would be random and maybe about 5 times a year.