Okay, so Zen's role is not to keep people topped off, it's to give them a buffer in the fight. His orb does 30hps, and as where you can use this just for healing in down time, it's more there to counter the damage they will be taking. Discord orb is what really makes Zen a solid choice (outside of that sexy ass ult), but not his healing.
If you want someone who heals well but can still be offensive when they need to be, get good and learn to play Ana, or maybe pick up Moira. Zen is perfectly fine where he is and I honestly wouldn't agree with any buffs they throw his way.