Right now a lot of PvE events are pretty similar. In the majority of them you’re fighting against the same set of villains and some various goons that place a bunch of countdown tiles. Because each event has similar opposition, you tend to play the same lineup for each event.
PvP is pretty similar. Sometimes there’s a mandatory character, and sometimes you have a roster restriction, but generally you can get away with using a similar set of characters from event to event.
Adding some events that actually change the puzzle mechanics could help increase the variety. Say there’s an event where no red tiles will naturally appear, or a event where you’re twice as likely to get black tiles. This would require you to come up with new teams that take advantage of the different color distributions. Or make an event where both teams start with a few strike/protect tiles on the board, which would favor teams that have abilities that interact with these tiles. Have an event where all ability costs are reduced by 2.
Changing the actual mechanics of the game for an event would make it so for the event you need to rethink how to take advantage of the different mechanics, and different characters could excel in the different situations. This would encourage having a more diverse roster instead of playing the same few lineups for the majority of matches.