Duo's Issue? Pfft.
If a chief concern is how to "balance" the map for Duos when it's made for Solo or Trios, it's simple: Don't.
Just make Duo's use the trios map balancing. When I tested with cross play off, it may have taken longer, but I was solo'ing trios bosses. It's not that big of a deal. (was GREAT loot btw lol)
That way, if we want duos, it's our own choice. "Note: PvE may be harder" or something like that.
I understand the splitting queues argument, but how much is actually being split if you factor the amount of people that just log off because their third isn't on right now and they don't want to 2v3vX because using party fill generally blows due to lack of coordination or comms.