Not sure if it has been raised before but as a suggestion, why not introduce set bonuses for teams made up of all characters from a specific set/edition (the little title under their name)
So for example if my team is made up of all “No Time Left” characters my team gets a defence bonus or crit chance bonus or something like that.
Sort of like leader bonus.
You could even work it like mods where part sets provide a slightly lesser bonus.
Could be a way to bring relevance to the non 6* characters that folks have kicking around?