OpenAI may have a model in the works that is similar next-gen size and architecture to Fable, but this isn't necessarily it. I'd guess that 5.6 was more of a hasty reaction to Mythos - same base model (same size, same price) as 5.5 but with additional post-training to make it more competitive with Mythos/Fable in some benchmarks.

Mythos/Fable is supposedly next generation in size vs Opus, and is rumored to have some architectural innovation in terms of dynamic routing/compute, possibly only fully enabled with Fable which at $10/50 is still twice the price of Sol 5.6's $5/30, but a big reduction from Mythos preview which had been an astronomical $30/150 possibly due to the dynamic routing not yet having been enabled.