Ziggs is "niche" in the fact that he can both sling out spells and take turrets quickly, but he's simply outclassed by Xerath (for slinging out spells) and Jayce (on both fronts. lmao).
Sona - Sona is a mage/enchanter hybrid which is why I included her here. No one can argue that she's god awful to play atm, however.
Vel'koz/Viktor are strong when people know what to do with them, but the learning curve is punishing and I know first hand that Viktor runs out of mana really, really quickly and that building mana items (like tear) on VIktor isn't really an option because Viktor really needs to upgrade his hextech core and tear sets him 700g behind on that front.
Ryze is just a mess, but is one of the champions I would rather not see anyone on my team play.
Syndra is just overall weak atm and really only annoying on ARAM where she gets to take clarity and mana suddenly becomes a non-issue.
Zyra - I would really like to see some meaningful Zyra changes/buffs because right now, I only really like playing Zyra on ARAM where it becomes "do NOT enter this zone where I have 7 of my plants or else you'll die just like these minions". He midlane is largely non-interactive because she relies so much on an AI with -50 IQ to deal damage to her opponent. Q hitbox especially is dreadful - it really only feels like a way to proc your plants rather than an actual, damaging spell.