TLDR: The balancing would be madness at this point.
One thing I’ve noticed Vainglory doesn’t have is a lot of reactionary silences, and having more heroes with aoe skills that are strategic would create situations where more of that is necessary, while also breaking the game for matches that don’t have multiple aoe support heroes.
What I mean by that is take reflex block; it’s one of the few reactionary skills that allows you to choose to directly influence an enemy’s ability AFTER they’ve performed it, because silencing before wouldn’t address more enemies having more opportunities to team buff/debuff, unless you had a global silence, which is brutal. Having more aoe abilities, especially boosts or weakens, etc., creates situations where we need more item abilities or hero abilities to shut these down AFTER they’ve been performed, because if more heroes have aoe support skills, it would also be unfair to let them all stack, but it would also be somewhat unintuitive to only let one boost from one hero apply to a team at a time. At the same time, it would ALSO be unintuitive to shut down all these aoe support abilities at one time, because enemy silencers would just wait your team to stack buffs, so the only strategic solution would be to have single hero shutdowns more available to pick and choose certain buffs to remove from a whole team, and that doesn’t even seem to be a mechanic that’s currently programmed into the game (that if a hero is silenced, their buffs are also removed from whoever they applied it on). Other problem with that is when whatever hero or item that can do that is going against an enemy team with only one aoe buff/debuff hero, where the frequency of shutdowns that you need against multiple heroes would be overbearing on one hero.
In short: Having more aoe support means having more ways to counter that support, but most of the dynamics of that are frustrating for strategic gameplay, especially when you put those counters against a single buffer/debuffer.
I wouldn’t be surprised if we got one or two support heroes that could perform multiple aoe buffs, but not a lot of them. That’s the only way I see it working out. Having multiple heroes with team buffs would just be gamebreaking in too many ways. It’s also why team buffs are usually so weak. It would also affect balancing team buffs against solo buffs, another area of complications.