The problem with this is that it would make synergies too powerful.
Yes, making Nightsister Acolyte viable should be something that is don't through Nightsister Synergy, but it can't make the rest of the Nightsisters overpowered.
Greedo doing an extra 30% damage is massively overpowered and bonuses like this would simply lead to one or two factions becoming meta.
Then, the problem of heroes who aren't viable in arena is only exacerbated.
If you take a look at this thread the devs have stated that they won't give faction bonuses like this, but will instead be making factions mean more by introducing new heroes with leader and unique abilities which greatly affect the faction.
For example they released Chief Chirpa who brought the ewoks from nothing up to a #1 contender in arena.
Unfortunately, the majority of these new faction booster heroes are chromium only, so it will do nothing to change the game for spenders and non-spenders alike.
