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*