NOTE : This involves my personal opinion. Please refrain from reading this post if you are offended from any of my opinions.
Giant Skeleton : Maybe buff the bomb radius, or the bomb load time. He's completly off-meta and considered trash as of now.
Dark Prince : More Health, or immune to crown control effects if the shield is on. Same to GS above, he's off-meta and is really outshined by other cards.
Clone : Cloning a unit fully instead of a producing a sheet of paper 1 HP, but with a set time for it to exist would be better.
Sparky : Do I need to say anything?
Ice Wizard : At least make him kill larries.
Tesla/Cannon : Simply outshined by other towers or buildings. Needs a decent buff in HP or Damage. Maybe if the Tesla hit two targets just like ewiz.
Mortar : Maybe increase the splash radius, or make it last longer. 30sec is too short.
Bats : Count rollback to 5 please?
Bomb Tower : Death damage like in CoC perhaps?
Goblin Hut/Barbarian hut : Simply outshined by Furnace and Tombstone.
Rage/Freeze : More duration perhaps?
Mirror : Another +1 level for the mirrored card (i.e. level 8 Mirros mirros level 15 commons and level 7 legendaries)
Skeletons : 4 LARRIES AGAIN PLEASE
Ewiz : Hasn't been nerfed enough. Needs another 0.1sec attack speed nerf.
Graveyard : 6 elixir instead of 5 elixir should be enough, in place for producing 3 more skeletons.
Night Witch : She's still OP. Reduce the bat production speed to 7sec, the same as a regular witch. Also reduce the bats on death to 2 from 3.
Knight : Used too much and pretty much counters every ranged dealer and suprise attacker such as bandit. Needs a HP -10% nerf.
Battle Ram : Overused and really hard to defend once it's combined with beatdown pushes or semi tanks/bandit. HP decrease probably?
Princess : Her main purpose is splash damage and log bait. Increasing her spawn time to 2sec or an damage decrease would be good.
Archers : Should be one-shotted by Logs and Arrows.
Goblin Gang : Decrease the Goblin count by 1.