It's a cool passive, so major changes are unnecessary, but Riot hasn't really been able to nail it yet. They've gotten close on both iterations, but not quite.
Morg's old passive was strong because it offered a ton of lane sustain.
Its problem was that it was spell vamp, so you only healed for 1/3 on your W/R.
Good in lane, terrible in teamfights.
Her updated passive is strong in fights since you heal for the full value.
Its problem is that it doesn't work on normal minions, so you barely sustain at all.
Good in teamfights, terrible in lane.
The end result for both is the same: Morgana is a pretty bad mid.
So here's my idea:
"Morgana heals for 20% of the ability damage she deals to champions and large monsters, and for 5% of the damage dealt to minions."
(W passive only triggers when you get a greater Soul Siphon proc).
She's still healing the same in teamfights, and she also has the same healing against minions that she used to have.
Doesn't really impact her supporting at all, but it'd do a great deal in bringing her back as a mid.