Ill chime in.
I own many fixed blades, folders and multitools and have carried and tried them all over the place doing everything you could think of trying with knives.
As far as realistic carry of fixed knives a Kabar, CS SRK or somthing of that size range is what I normally carry.
Folders... I just pick one I like that is easy to maintain and use. Emerson or Benchmade.
Multitools... I carry an issued surge most of the time.
A starpening stone, ceramic stick or diamond hone.
I usually carry two of each in my bag so when I get to where I am going I have a back up knife to carry if I lose or break one out doing work.
If I was to be a designer of some sort of multipurpose knife for the military I would shrink whole the OKC3 Bayonet down to a 5-6 inch blade maybe even 4inches (Think Cold Steel SRK size). Make it a single guard style so you can choke up on the knife and get rid of the locking plate on the butt make it solid.
I would keept the design of the scabbard for several reasons. 1. the knife can be retained at all angles if you have some of the blade inserted 2. it can be used for left or right handed 3. its MOLLE capable ( I would change it a bit and make a quick attach belt loop to) 4. it has a honing rod built in 5. Its very weather resistant
MY .02