Since it usually takes more skill to have to kill more people will their be a competetive gamemode where it has one of each hero vs one of each hero with each team having all the heroes?
Trying to coordinate a team of 9 is already enough of a shit show. You want to have teams of twice that, 18 people on each team that you have to try and control and will only get bigger as more heros are added? There is also the fact that you have to get 36 people together at one time that want to play different heros and are decent at them to scrim or play a game, that's literally more people than you need to do a 25 man raid in wow which is an mmo. It won't work.