In the mid game, you want to find Kennen as your carry. He synergizes very well with Keepers like Jarvan, and after this point, level up further to find your 4 cost units.
Additional comp tips
Sunfire Cape should be the first item you build as it is an early game item. You should use it in order to win streak into your level 8 comp. If it's already late game, you can instead build another defensive item like Dragon's Claw or Bramble Vest on Riven depending on your lobby. Or, if you get offensive items, you can build something like Blue Buff or Rabadon's on Riven to deal more damage instead.
In an ideal world, you would find your Chosen Dusk as Thresh, Riven, or Cassiopeia. Getting a Chosen Vayne will be fine, but won't be as useful as finding a higher cost Chosen unit.