[Threshold]

Feel free to have "No levels" as a requirement, but you are making a mistake when you say "they are an OOC concept."

Levels are extremely realistic and have tons of real world analogues. I will give one example and stop there, for the sake of brevity:

E-1 - Private E-1
E-2 - Private E-2
E-3 - Private First Class (PFC)
E-4 - Corporal (CPL)
E-5 - Sergeant (SGT)
E-6 - Staff Sergeant (SSG)
E-7 - Sergeant First Class (SFC)
E-8 - Master Sergeant (MSG)
E-9 - Sergeant Major (SGM)

(yay! time to remort and become an officer, or pick my epic class, or some other game-like similar thing) =>

O-1 - Second Lieutenant (2LT)
O-2 - First Lieutenant (1LT)
O-3 - Captain (CPT)
O-4 - Major (MAJ)
O-5 - Lieutenant Colonel (LTC)
O-6 - Colonel (COPL)
O-7 - Brigadier General (BG)
O-8 - Major General (MG)
O-9 - Lieutenant General (LTG)
0-10 - General (Gen)

I understand why some folks don't like levels. But saying they are "OOC" or "unrealistic" is patently absurd and false.

If you are listing requirements for RPIs, you don't have to justify the reasons. If that is a requirement, then just say:

4) No levels. We don't like them.