Agreeing on something would be a great start.
The deck is stacked, but not so much that a reversal of OTL's outcome is impossible.
Also note that while the Mu'tazila disappeared as a school of thought in its own right, its Ash'arite opponents had to incorporate a lot of the rationalistic methods it used, in defense of "traditional" conclusions.
So, a "middle ground" was possible historically.
In my view, the conflict at its core was not so much between reason and tradition/revelation, but between individual opinion and communal consensus, both elaborated through a varying mix of reliance on rational methods and religious hermeneutics. The overarching worry of Muslim religious scholars was to prevent discord in the community at all costs, a point where, by the way, traditionalists, mu'tazilites and ash'arites all agreed.
Not having a Caliph of dubious orthodoxy such as al-Ma'mun trying to enforce a core mu'tazilite tenet as official doctrine would have possibly helped to reach an overall consensus closer to mu'tazilite ideas in some critical areas; jailing your opponents hardly helps making people believe in the rational soundness of your views after all.