You have Cannon-Teemo, Bruiser-Heimer and Tank-Sion!
and Akali.
It's about after the mid-game
Then a Cannon like Teemo or Akali can be colapste upon if it farms a lane for longer then 40sec.
Heimer can have trobles aswell, at least he builded a bruiser and Ryla, to splitpush farm some, Teemo have movefast.
Still Sion-tank-support can splitpush twice as safe and effective.
So if you not plitpush or waveclear, you need to do other things, like Teamfight contribution, vision, objectives, tacticalfeeding or other things!
Karthus have a simple job, that spamming his ultimate when it comes off-cooldown, it makes him functional! his other job is to fill time, with team-walls and e-waveclear.
For akali this job is dam hard! needs huge skillcap to do so, and it may need to snowball harder then Heimer. its pretty Risky
Think Ekko better for you! he have same skill-cap-potential
but his q can do team-contribution, if you are to behind or lack skillcap