2 ideas here:
Dual-mono RackFX engines:
Having the option of either to split the stereo RackFX into two dual mono engines of the same effect would open up a new range of possibilties.
You could, for example have 2 mono instances of DynEQ4 or MultiBD on separate input-channels (2 vocals or maybe a vocal and bass,…).
Dual-mono mixes (Aux/Groups):
It would be awesome if you could split a stereo group into two separate mono groups.
That way we have the option of even more creative bus processing when mixing, for instance, instead of having a stereo drum group and a dedicated stereo drum parallel group you could have 1 mono kick group, 1 mono snare group and a stereo group for toms, OH’s and cymbal-mics. The options are many!
For auxes, this would essentially give you more monitor mixes!
Since it was possible to split the 3 stereo matrices into 6 mono matrices, this concept does sound feasible.