Honestly; it's just not that interesting of a kit on paper; besides the E, it's just 'give stats to an ally', and there's not really much that is meaningful about it besides who you give those stats to. Maybe when you post the stats, but it doesn't look like there's that much to go with on paper.
And you mention her building as a tank, but none of this suggests 'tank' to me, especially without numbers. A tank's job is to soak up damage, make the enemy waste cooldowns, and disrupt the enemy team, but none of her skills do this; she just gives stats to her allies (They aren't even defensive stats besides the health) and sometimes does damage to one target for a slow. That's not a tank, that's Sona without Aura's (Sorry if that's inaccurate, Sona fans). At least Taric gives his allies armor based on his own armor.
Even if her buffing skills became stronger for building tanky, that's a kind of Character that Riot doesn't want: a champion that is able to keep their team alive and undoes any poke or engage while simultaneously being unkillable is incredibly frustrating to play against: You can't stop their team because Medic keeps them alive, and you can't kill Medic because he CAN'T be killed on account of being so tanky. Probably the only reason Taric gets away with his own sub-par heal is because he's due for a rework anyway.
And you really should at least try to think of a passive. For some champions, their passive outright defines their playstyle (Gnar's Rage Gene, Evelynn's Shadow Walk, Rumble's Junkyard Titan), and you can honestly do some interesting stuff with it.
And yes, Soraka is still the dedicated healer; having the Q heal her is part of her gameplay yes, but that is hardly her defining trait. Soraka can fully heal a fatally wounded ally in a matter of seconds; I've personally kept myself and a near-dead ally Annie on my team alive for damn-near a minute with the enemy Zed and Zac (I think it was Zac) relentlessly attacking us up until we got back to the tower. (I think Annie even got a kill out of the whole exchange)
