The only way I can see this happening is if you have the unusual circumstance where the Iranian Plateau is divided, but the Mesopotamians are taking their coastline - and acknowledges that it doesn't have the strength to unite the Iranians, but it could either
1) Regularly raid them to destabilise the region
2) Set themselves up as a regional policeman. Purposely making it so that the Iranian people can't unify, but working alongside them during nomadic invasion - so essentially turning it into a series of tributary states. A bit of effort to ensure that the different proto-Iranian groups form distinctly differently.
Problem is that you have a Mesopotamian power expending resources on the Iranian plateau, that may not be profitable, when they could preferably displace, colonise, or assimilate the region - which would lead to a non-Iranian power on the plateau. (Is that acceptable?).
A third/fourth option is to add Central Asia, and India into the mix. Have strong polities there try and exploit the Iranian plateau. The Central Asians raiding and setting up tributaries, the Indians using the Iranians on the Plateau in a proxy war during a trade war with a strong mesopotamia (don't know why, but possible), and you have 2/3 client states in the area. At the very least the Indian and Mesopotamian powers can use them to protect against powerful steppe tribes.
So maybe that is the trick, have a steppe tribe charge through and lay waste to the Iranian steppe, and overambitiously try and raid Mesopotamia and the Indus Valley, forcibly conscripting the local Iranians into forming levies for them, providing the majority of their strength. The local powers suffer, but push them back brutally, and each draw the same conclusion that they want a client state as a buffer - using Iranian soldiers to protect these client states, and possibly drawing on them for troops in their own campaigns, reducing the populations on the plateau.
Another option would be to have the steppe tribe (and retained levies) that survive, only to be broken by some settled group in central Asia(the Bactrians/Proto-Bactrians?) who bring them into their protection, using them as a cavalry arm for their forces, to unite central Asia. - Then you have three powers, neither of whom wants a strong Iran, using the plateau as an impressive buffer zone. None of them wish to spend the resources to conquer it all, as it would end up being too wealthy and cause issues with their own power structures, be too costly to take on the other powers, and would rather use Iranian men and money to expand closer to home.