Hmm in all honesty i think her ult and passive just needs to be reworked completely. Her ult is completely single target based which loses power later game since when behind or even ahead syndra doesn't have the power or means to getting a kill or getting to people. If she doesn't get ahead she becomes completely lackluster without leaving a huge impact even with a full combo. On the other hand if Syndra gets really really fed her ult can 100-0 without much counter play. Her passive which does make her late game more manageable most the time on a bad game syndras can never get the complete effect by just having a rough lane making the passive pretty useless on occasions. I know this doesn't apply to high elos who can farm well however, if the game runs short or a gg at 20 minutes syndra will never have felt the effects of a level 18 and all of the passives strengths. Even current meta mid laners Viktor, Veigar etc just completely out scale her to due to this querk. I believe a great way to solve most of the problems would be to make her passive an active for her ultimate along with the ultimate giving cool down reduction and movement speed sorta like a singed ult. This would solve her problem of only receiving her passives power very late game while also making a bit more counter play by not having the higher radius of e , more q damage and the increased slow duration on w in the case she makes it to late game and can 100-0 anything. For her new passive i believe it should be changed into something that promotes landing a good combo. Current syndra doesn't have to land her full combo necessarily and can still 100-0 people when she is very very fed (which is rare that will happen these days). The new passive could be after landing a q w e combo for the next 10ish seconds her dmg is amplified by 15%. This promotes her to have the skill to actually land a full combo to bring out her full damage potential. Along with the combo damage amplification syndra should also receive a bit of love for spamming in her landing phase. For this i believe every 10 cs Syndra replenishes 10% of her maximum mana at level 1, 12% at level 2 ,14% at level 3, 16% at level 4, 18% at level 5, 20% at 6. The passive stops scaling at level 9 where the laning phase ends and has a replenish of 26% after 20 minions. This allows her to spam more in lane and allows her to be more passive and get through lane by sustaining (since syndra has quite a few counters)
Recap:
Syndras New ult (UNLEASHED POWER)
Upon Activation Syndra receives increased q damage, increased w slow, increased e cone radius, movement speed, and cool down reduction.
Syndras New passive (TRANSCENDENCE)
Upon landing a complete combo (Q W E ) Syndra receives 15% damage amplification for the next 10 seconds.
Syndras New passive (DARK SIPHON) upon killing 10 cs syndra receives 10% percent of her maximum mana back at level 1, level 2 12%, level 3 14% etc. At level 9 this passive caps and after 20 cs syndra gains 26% percent of her maximum mana back
I know for sure this needs some tweaking especially Dark siphon but over all i feel these are very concrete and solid changes and let me know what you guys think! Im very open to suggestions!!!