Oh, I see.
I assume Ezreal ADC? In this case, what you want are more specialized champions. Ezreal is fantastic in that he can poke and siege at long range, he can burst with ability spam, and he can even scale late game pretty well, but he can't do every job perfectly. If you want to complement Ezreal, what you want to do is start using ADCs who are really, really good at doing very specific jobs. Graves is a pretty good Fighter and Burst ADC, whereas Kog'Maw and Vayne are godlike late game tank shredding carries, to get some examples out there.
As a projectile Mage, you're going to want a non-projectile Mid Laner to use against Tanky teams, or teams with Braum or Yasuo. Xerath for example has an array of powerful AOE effects that can't be blocked off by most tanks or projectile blockers, and he has the long range to keep him safe regardless. I'm not too familiar with Mages to be perfectly honest (I'm primarily a melee Assassin player in mid, hehe) but I do know for a fact that when dealing with a champ like Ahri, hiding behind tanks and being out of her range greatly hinders her offensive presence. Having Xerath who has a greater range and is less easy to block than Ahri would do wonders in rounding out your mage gameplay.
Jax is similar to Ezreal in that, depending on how you build Jax, Jax is capable of doing a hell of a lot of different things but is not the best at any particular thing. Again, specialization is the key to rounding Jax out, being able to find champions that do very specific jobs very specifically well. Jax is seen both as a top and jungle champ, so I won't call any particular champions out by name, but again, you're going to want to find a champion that is godly at doing what they're designed to do.
Lux is an odd little utility mage. With two mages in your repertoire though, I imagine you don't play Assassins very heavily? May want to look into that.