I don't game in Death Fields per se and don't have anything to do with 40K, so for my purposes a main army box (or occasionally two, as in the case of the Cannon Fodder set), a heavy weapons/command box, an accessory sprue, and maybe an occaional light vehicle kit is fine by me.
I don't think every faction needs all of the above - if's OK by me if some factions never see e.g. a command/heavy weapons set, for example. I wouldn't mind seeing the lizard-men get an accessory sprue of additional weapons types (fantasy and sci-fi alike), but I don't think they necessarily need a fully-fleshed-out suite of vehicles, heavy weapons, command, etc., unless Wargames Atlantic are really inspired to do something like that.
Nor do I think any of the above needs to be specific to any particular faction - for example, if a box of generic power armor or a set of motor bikes is produced for general-purpose use, which can be customized to different factions, that's fine with me.
General-purpose accessory sprues that could be used to convert historical figures into factions with different styles of sci-fi weapons or non-weapon gear are fine with me - I think the first accessory sprue includes some Cannon-Fodder style rifles, some Germanic gasmask/helmet combinations, and some odd laser-rifles - it doesn't fit in with any specific set, and that's OK with me. I wouldn't mind seeing more stuff like that: alien heads that could be added to any Death Fields faction to reinforce them with aliens, for example.
Really, I think the Grognard command/heavy boxed set supplies some great heavy weapons that can be used by any faction, with a little creative modeling for the gun crews - it's not necessarily a Grognard-only set (though the figures certainly make better Grognards than anything else!)
If some factions seem to sell better than others and get a little more love than others in the form of extra accessories, special weapons sets, vehicles, or whatever, that's fine by me.
I'd hope that any Death Fields rules are flexible enough to allow for gamers to source their own accessories and armies and figure types, and plug them in with appropriate plug-in rules. Found some great figures that would make a cool psychic team to the Grognards or a lizard-wizard to the lizard men? Or, did you find a collossal spider to add to the giant spider faction? (Reaper makes a great collossal spider!) Maybe someone has an idea for converting some fantasy giants to support the sci-fi British or Raumjager. There should ideally be allowances in a hypothetical Death Fields rules for plugging these sorts of specialist figures, even if WA doesn't produce specific models for them - let gamers use some creativity in sourcing and adding their own models and conversions, with Wargames Atlantic kits and bits to make things easier wherever some opportunity strikes (an accessory sprue to ease conversion of fantasy ogres or giants for sci-fi games would be nice, for example.)
And I don't easily think of a Death Fields rules set as a complete, exclusive universe of its own, but rather a toolkit for building sci-fi stories of our own: a crew of sci-fi marines battling giant spiders on an unexplored colony world "bug hunt", or zombies vs. apocalypse survivors, or little green men vs. a rocket-ship crew, would be as much a Death Fields game as a formal Grognards vs. Bulldogs fantasy sport grudge match. Some of these teams lend themselves better to a formalized "complete" team than others, and that's OK!
Give me one or more interesting kits, and a flexible enough rules set to support my imagination and whatever additional models I can source on my own, and I'm happy.
In short, I don't think there needs to be a formal definition for a "complete" team, and we're better off without each faction having a built-in checklist of unit types to check off the list - let gamers sort out what "complete" means for themselves, and supply great model kits and bits as demand or inspiration strikes Wargames Atlantic.
But, that's just me - it's likely a Your-Mileage-May-Vary situation. I'm definitely a different sort of sci-fi gamer from the customers who are e.g. shopping for an alternative source of 40K models, who may need a complete selection of faction niches filled!