I dunno exactly how'd they look without their fancy gadgets, but I think they'd look something like Combine Soldiers without clothes on
the problem with this is that it's like saying 'this is what the roman empire looks like' and then posting an image of a spearman from western europe without knowing really what anyone else looks like.
i'm willing to say it's a safe bet they'd look nothing human-like. i'd use a few cut enemies as an example like the 'combine alien soldier' or the 'alien assassin' as examples, but those were cut and there's no real way to say whether or not they're canon, but i guess that's part of what my post before said- interpretation.
what we do know is the combine has advanced so far that they're able to rely entirely onto technology, take for example the advisor. we're able to also know that the advisor is not at all the combine, but maybe only a higher 'class' of conquered species, seeing as they seem to run the show with some of the forces on earth, kind of like a commander or deployment leader, though that much is left to speculation
near the end of half life 2 when breen was trying to escape from gordon on the citadel, in order to transfer over to a 'host body', when the portal to an offworld (also mentioned by the overwatch voice at one point) location, we see an entirely non-earthlike world, red, glowing skies, and multiple citadel-like structures, and that's about all we can really tell. this probably wasn't 'the' homeworld, but it does give a glance at what other worlds under union ownership looks like