I think I'm going to participate in voting, but personally I'd suggest you to do a little bit of each.
Option 1.: 16 companions based on a not SOD story, adjusted to fit into the sod world?
This can be quite effective, both medieval mythology and modern pop culture offer an endless amount of figures you can exploit, but I generally advise you to add something extra to the basic material. Ie, a Lancelot-like knight who is (literally) obsessed with... ahem, *engaging* with the opposite gender... or make Cluny the Scourge (from the Redwall series) into a human mercenary... etc. you get the idea
Option 2.: 16 companions based on my own experiences in SOD
Well, personal experiences can vary from player to player, but if you've got a good idea - say, converting the character you used to beat SOD Gold into a Companion with enriched backstory and personalized equipment - don't be afraid to use it.
Option 3.: 16 companions based on the factions, both normal as mercenary as bandit
I tended to use this approach in the past for my own works but realized that unless you pay close attention to personal characteristics of the Companions you'll end up with a bunch of highly stereotypical figures (ie, all Antarians are righteous plate-clad paladins, Marinans have the best scientists/engineers, etc). This approach can be a source of inspiration nevertheless just watch out how to use it.
Option 4.: 16 companions based on nothing but what the forum people suggest
This is a double-edged blade. Back when I spent some time working on Companions for the "Revenge of the Berserk" mod (it was an open 'competition'), I perceived that while some people (usually roleplay fans) can and eventually will come up with some astonishingly good ideas, the majority of character ideas often turned out dull, unfitting to the setting or lacking many details. Then again, those few good character designs which surface in such a competition are like precious little gems...
At least that's what I think.