To just put into a retrospective, one of the closest of mechna is the AT-ST which is a bipedal reverse joint vehicle operated by two pilots. Several other examples like the AT-PT give a little argument but just by view, the results remain the same. Very primative, prone to tip overs and very little firepower. Height varies but never taller than a two story building.
General rules from what I believe to be Mechna standard in star wars is this: No true control of arms and lack thereof, weaponry is simplistic and height is very limited. Granted the last one can be argued since a Super Battle Droid 3 has a artificial gravity generator to keep it standing but remains the same. It needs a gyro to keep standing and an engine in the center core.
Put it lightly...they be expensive to attempt things beyond the formula which would require to my thoughts, at least dozen different factory submissions for various components. I have wanted to make my own but had to Pace myself with a project here or there. May restart it even now that I think more of it.
If you want to look at something simple to an evolution of a vehicle, look up Earthsiege and see how theirs compare to others. Very primitaive, no arms but the idea of armament exists. Another source though more advanced can be Battletech. Locust, Cicada, and even the Hoplite are canidates. Long as they have barely and real motive control, it is not that all advanced.
However as I stress...no arms, minimal weaponry and hard control of said mechna with height being regulated. Any improvements towards it should be a factory submission and would take time to build any form of project.
Oh and to stress also nearly due to forgetting, everything mechanical is used to control it. No fancy Neuro helmets, goop or otherwise. Switches, levers and joysticks. Take note of it from Episode VI inside the AT-ST.
All out of steam now, bed time.
Playing the bad guy here. While I honestly believe we would never get to Gundams or anything close to it, I do believe some forms of mechna can exist but in extreme basic form.