A very good question and I would say your assumptions are very much aligned with what I had in mind. I would agree that Joker, Hero and Banjo are not gunners in an archetype sense, but do possess some zoning options. I would also agree that a gunner-type character would be constituted by one that:
-Possesses at least two or more projectile specials and/or smash attacks that are key to their playstyle
-Possesses projectile normals a la Megaman (lemons, Up Air, Dair, F Smash) and Mii Gunner (Fair, Up air, F smash)
Stereotypical gunners on the current roster include, but is not necessarily limited to:
and maybe
. If Geno had actually made it into Smash, he would have fit perfectly into this category as well.
Granted, these aren't rigid requirements or intended to be fan rules of any kind, but more of the typical description of the gunner archetype. I understand that the lines can be blurred and some characters can fall into multiple categories, but it is interesting to note that the stereotypical gunner archetype is the least represented of any other fighter type, though zoners in general are fairly common. Again, to your point, KOS-MOS, Doomslayer, Master Chief and pretty much any FPS character would fit this description fairly easily. As for Elma, that would greatly depend on how Sakurai would choose to represent her since the fighting style of XBCX characters can be pretty fluid.