Strange request. Without knowing your family or people personally important to you both, there is absolutely no way to do this right.
Typically a middle name is given in honor of someone that has passed or eventually will pass, so the name carries on.
Look back to both of your families and see if there are any nice or unique names that you could re-use.
My suggestion is you consider doing that.. instead of just finding a unique cool sounding name. (If you end up doing that, let us know).
That makes sense. I'm not sure we have ever done much of that.
Hopefully we will know who they are when they come out!