Soraka isn't actually that op, she is simply a great fit for solo queue

so I'm not gonna argue that Raka might need some nerf, but I think that Raka's high win rates have more to do with how good of a fit she is in solo queue. 3 simple reasons: she doesn't require heavy team coordination, she has team comp versatility, and her kit encourages positive habits

Raka is like the opposite of say Yasuo or Kalista: she does not need her team to coordinate around her, and instead of requiring other teammates to set up her plays her entire kit tends to be about making other teammate's plays easier. 2. Though she is not as versatile as thresh, the role of "healer" is a good fit for practically any comp, meaning she is naturally a safe pick on all levels of blind. 3. I suspect that her literal lack of damage and healing/sacrifice kit tends to influence player psychology towards good solo queue habits. Namely it discourages selfishness and rewards healthy amounts of cowardice.

So with that I think it's kind of obvious that Raka, even while balanced SHOULD predictably have a higher than average win rate across elo's, whereas if say Kalista had a similar win rate you'd know something was wrong. (Also her win rate wouldn't be as high if people just treated her with the same priority you give adcs in a teamfight)

That said she's probably still stronger than she needs to be so suggested fixes that wouldn't ruin the fun:

1. Increase health or mana cost of repeated heals over a short time frame (forcing Raka to take a break from constant healing in teamfights) (not sure if this one actually really dumb because it encourages passive play and takes some of the fun out of one of the gunner parts or Raka, namely frantic healing of teammates that in team and duo fights)

2. Increase the cost of healing significantly while decreasing it if starcall has been landed recently (forcing Raka to interact more with the enemy)

3. Decreasing the fun but insane healing received from hitting multiple enemies

4. Decrease base values of heal while increasing so ratios. At the same time nerf her early game health regen, essentially making a hyper carry support (and justifying her power with a painful early game) her ult gives enough free assist money that she can definitely withstand being forced to incorporate heavy as items into her kit

5. Decrease the impact of a single heal, making them have to ramp up like new mordekaiser but less insane

6. Make grevious wounds effect healing at least a little, like 25% would be enough to make a big difference to a Raka