By level 6, you should ideally have a Chosen Cultist by this point. This allows you to have 6 Cultist along with another synergy to run. Kennen will be the best mid game addition even without items, so look out for him.
Additional comp tips
You need a Chosen Cultist in order to complete the team at level 8. It can be any Cultist really, but you may want to sell an early Chosen Elise in hopes of hitting a higher cost unit like Aatrox or Jhin.
Kennen is a very potent mid game carry that synergizes well with Elise and Aatrox. You can use him as a solid transition unit for the comp.
You can opt to try and go for 9 Cultists if your Jhin and Riven items are sub par. 9 Cultists isn't as strong as a good well rounded team, but it can be an option if you happen to win streak into it.