Another issue (other than her AP ratios which are thankfully being changed next patch) that I have noticed about Syndra is her kit doesnt allow her to be very viable for team fights. The biggest issue is her Ultimate, which is what im specifying. Her ultimate targets 1 enemy and blasts them with all her spheres (be sure to have conjured up 2 to 3 spheres beforehand..because you know that you have all the time in the world during a team fight >.>). It pretty much is a single target large burst ability...which simply cant compare to other AP mages' usefulness such as Orianna's Knockup AoE burst ultimate, or Anivia's DPS radius ultimate that is on a TOGGLE to boot. You can even consider the Queen of group control herself, Zyra with her devastating ultimate, lovely thorns, and one painful as heck passive.
Let's not forget other mages that also have a single target focus ability, however they offer more to the table alongside it. Consider Malzahar whose Ult suppresses a target down, making him very useful to shut down a target. Or even Veigar, Though his Ultimate is also a 1 target burst, it scales off of his enemy's AP aswell, making him continue to be useful late game. Other than their utlimates, these two mages' kit also offer a variety of group control and shutting down enemies. Veigar has perhaps the largest group stun ability in the game, while Malzhar has a very large AoE DPS ability that does damage based on the enemy's maximum health. Dont forget he also has a silence. Syndra simply cannot compare, as her slow effect is hard to land and minimal in effect aswell as her stun cannot exactly always be timed right for group combat.
My Idea for Syndra is this:
When her ultimate strikes at the target specified each sphere that strikes the target it releases a shock-wave small AoE pulse. This shock-wave would do damage to all enemies around the specified target, making her more viable for group assistance in general. Each Sphere that is added to the ultimate would add to the damage, and affect all enemies surrounding the target. This change would make it useful for any Syndra that is attempting to take advantage of as many spheres as possible to utilize (perhaps at baron?).
With this change, Syndra players could also target a certain individual in the group in an attempt to also maximize the damage potential done to those surrounding the target. Thus it would give more options in general to its usage.
tl;dr: Syndra in her current state and kit isnt very viable for team combat and there are much better picks. I suggested an idea in the bolded section.