What's your definition of useful? Zed and fizz are both assassins, so I take it you like deleting people? Assassins are feast or famine, if you're looking for something more forgiving in the event you feed you may want to consider a utility champion.
Annie is a good start. An absolute terror if she does well, and a reasonable stun bot if she falls behind. A 2-3 man stun with the bear is pretty much GG if you have the follow up. She can also fill the support role in the event you ever have to fill.
Ahri is also a good choice, who shares assassin-like playstyle with zed and fizz. She can sustain well in lane because of her passive and be incredibly safe if you kite well with her Q and ult. While she suffers if she falls behind, if you land a charm on the right person at the right time you can easily turn around a difficult fight.
Lissandra is also fairly simple, though somewhat immobile. Her poke makes pushing lanes and applying pressure very easy to prevent the enemy mid from roaming. Once she is 6 she has insane turnaround potential with her ultimate to remove someone from a fight or to survive things like tower dives.
My last pick would be Orianna, who is probably the most complex of this list. I find her personally quite easy to pick up as her skills are deceptively simple. She provides amazing utility for your team in the form of shields and MS boosts. Her ultimate is a huge presence and while the damage on it is nice if you're ahead, sometimes just setting enemies up for your team to obliterate them is all that's needed to help your team.