The increased number should see more variations in play and HH can justifiably up the difficulty significantly as well for more fun. Also gives more reasons to care about keeping an overall strong team apart from your top 5 (just like for Wars).
The numbers are not fixed but that’s the general idea I’m proposing to keep HH fresh since 3 and above players for Co-op has a lot more complications tech-wise so upping the usable heroes with the same amount of players to counter that.