I don’t know about you but I have a lot of 5* heroes sitting at 3/70 waiting for mats. How about a tournament or event where 3/70 is the max for eligibility?
-norns
-uraeus
-fenrir
-cobalt
-vivica
-reuben
-bertila
-zulag
-inari
-horghall
-kaghan
-richard
-leonidas
I’m sure other players have more than that
Have you been using them for lower levels of ninja tower or taverns?
NOT practical bcoz, everyone may not have adequate 3.70s to participate in those type tourney,
BUT
a more practical FUN possibility can be if 3/70 are allowed in 4* tournaments as they haven’t become full blown 5*s yet…
Instead of an awkward and restrictive rule like 3/70 I’d rather use a team cost limit. It opens up a lot more possibilities and doesn’t punish people who prefer to either max 5* heroes or leave them altogether and max more 4* heroes instead. Also, a mostly useless game component would suddenly gain importance.
If there is a team cost limit, say 90 TC for instance, players have to judge whether they want to use two maxed 5* and have only 38 TC left for the remaining three heroes or focus on 4* and 3/70 heroes to have a more balanced team in terms of strength.
I think this might lead to interesting and maybe also unexpected team formations. Maybe we even see a few top 3* heroes teamed up with maxed 5* 
I was proposing the idea. Unfortunately, Small Giant will not implement such feature, I think, as this will only show and accentuate that ascension mats are hard to come by. On the other hand, it will also show that 5* heroes are not hard to get.
Either way, it should be a fun event.
I would support this idea which makes a player think…plan…