Since a lot of players were not playing or just didn't know the game could've missed a couple of ''Exclusive'' units.
So what I suggest is to maybe rerun all the raid units for the anniversary update but make it so players can only get it once and only make the unit droppable from an EX stage with a 3% chance,
I think it could be a good idea to bring old players back or help people get units they could've missed back then, the EX stage would only give the unit as a reward and maybe some gems.
Feel free to add anything to the idea if you feel like it.