Despite still having not managed to unearth my outline sheet for "replacement" miniatures for official Defiance universe troops, and the fact that (contrary to the doom-and-gloom link above) the Assault Troops do seem to be coming out after all (three cheers for mr. Drage!), I might as well start gettin' this show on the road...
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Namely, was perusing Mongoose Publishing's site and noticed that there are some new releases with Defiance potential on the horizon. Namely, Skinnie Raiders, Skinnie Soldiers and Cliff Mites.
The two former ones are IMHO pretty good for true Vobian troops (Specters and Phatoms, respectively), while the latter would go well for Vobian Phase Spiders. All are reasonably priced sets and, I believe, plastic so they are fairly easy to work with -- as some work will be required. EDIT: while I haven't verified this, it does appear as if the Soldiers are in fact single-pose metal miniatures, which does throw a bit more grit into the gears, but shouldn't produce insurmountable problems.
Cliff Mites need some weapons, preferably biomechanical looking ones mounted on their carapaces. Unfortunately the availability of such weapons is rather limited. Excelsior Entertainment may have had a few in their Warzone extra weapon sets but they've probably stopped trading after loosing the Warzone licence earlier this year (haven't contacted them lately tho, so could well be wrong). The best bet might be Tyranid biomorphs sprue from GW, it's listed as £4.00 per and has 3 "laser rifles" and 6 "laser pistols", plus a few left over gribblies an enterprising person whips up a "ghost ward" or two from in no time (I'm sure).
Skinnies, especially at 30mm scale, are quite too tall to be really good Vobians (one might be better served by using some 25mm scale minis with 28mm ones). They are sufficiently oddly proportioned to appear not-quite-human tho.
Aside of the size, which can't be remedied easily, the most glaring discrepancy is the head. This is a problem with Raiders, as they appear to have open helmets only; the Soldiers seem to have the closed helmet option. I suggest replacing the heads with Pathfinder helmets from the Mobile Infantry range, these are $0.15 a piece so that shouldn't kill ya. The original Skinnie helmet can be used to decorate the Pathfinder helmet, or as a backpack as the Raiders appear to have none.
As to the weapons, Constrictor Rifle will go for the standard Laser Rifle quite nicely, and the Raiders have pistols that can be used for Laser Pistols, but the rest of the weapons will be a little more problematic. The Constrictor Cannon should be fairly easily to convert to Gatling Laser and the Neural Beamer would make for a Ghost Gun (although you'll have to get both Raiders and Soldiers so you can mix and match the weapon options). The AT and AV Launcher are less easy, but Dark Reaper launchers, again from GW and not too badly priced (£1.50 each), might do the trick nicely enough.
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All of the above said, I blithely used GW Eldar troops as Vobian Specters and Phantoms when playtesting, and am still alive (if only barely)...