The current Yoda event just states "Normal" in the difficulty level. How about a "Hard" Yoda set of events. Once you complete the normal set you get access to the Hard set. However the hard side allows you to use any characters you like, and instead of Yoda shards, players can earn a special character like Emperor Palatine or Jedi Luke Skywalker. There should be a different character for each month of the year. So if you do the event in January you get Jedi Luke, if you do it in March you get someone else, and etc... This way there is always a new person to try and earn.
As for difficulty, just increase Yoda's level by 10. So the 7 Star event would be a level 100 Yoda. 6 Star level 90, 5 Star level 80, 4 Star level 70, 3 Star level 60, 2 Star level 50, 1 Star level 40.
This would extend the life of the event and make it very useful for players who have already completed it. Just do not change Yoda's levels if there is a cap increase, and keep the difficulty the same. So those that take the time to get higher levels will find the event easier.