To make an actual faction (even just as allies) with either, they'd need multi-part kits with options. Otherwise, appeal is going to be pretty low.
DW are easy, just do a sprue and pack it with existing stuff. And a clampack sprue. Boom, done.
GC would be a little tougher. Characters are all monopose without any real options nowadays anyway, so they can make new sprues with those and package them in clampacks. It's the hybrids that'd be the bigger issue, but theoretically having the base design already in the computer would mean they'd just need to come up with more weapons and maybe alternate types of units, then design those, reposition some stuff, design sprues with the bits chopped up as needed. Certainly not impossible, but I imagine it'll take a bit of money to do all of that. (If they're trying to gauge interest for a 40K faction off of a board game, though, that'd be a bit... silly.)