Mistral WB that is essentially a massive hub for traders and customers alike, being able to buy goods and services alike, even if they're not "publicly available"?
I'm not sure what you're asking. If you're asking if we're going to make a full WB post for Mistral, then no. People have to go off of what the show gives and the small supplemental material we made
here. Though I will be moving the location of Mistral on the map.
Manned bi-pedal walkers?
Like a Paladin? You'd better have a good reason for having it and it'd better not be as good as a Paladin. Atlas is the upper limit for raw tech.
Multi-form prosthetic? (Think Deus Ex)
Unlikely, you'd need to explain it really well. In canon, Mercury's were just legs and he still wore shotgun boots and Ironwood's arm is, as far as we know, just an arm. In our canon, Alapelagias has the me "advanced" prosthetics but only in the sense that they're also weapons.
Do kingdoms (apart from obvious Atlas) have armies? I would assume so because Atlas Military Airships seem to have weapons designed for use against people, rather than Grimm.
Mistral, Vacuo, and Vale all have militias, but not a proper military. They all used to have military's in canon and site canon but they all dissolved after the Greyed War with the exception of Atlas.
Nuclear Weaponry and Nuclear Power Plants.
BEAR WITH ME ON THAT LAST ONE.
If we assume that Remnant is pretty much Earth with magic and shit, we can assume that elements are similar too. Since we have MANY nuclear isotopes available to us, wouldn't it also be assumed that nuclear fission has been achieved, at least in the use of power generation?
Hard no. As far as we've been shown, it's all Dust. Until we're shown otherwise, NO nuclear anything. Nuclear opens a whole can of worms the WB team is not willing to deal with.