I am by no means anything approximating a creative person, but I guess I can point to a few designy things that I like, and tend towards what I would imagine here.
I kinda like the retro-futuristic stuff you can see in the first two alien movies; you know, the stuff that they thought was really futuristic at the time but is just laughable now? Prometheus kinda bridged that by keeping some of the overall design the same while throwing out the stuff that obviously does not fit. However I do tend to prefer the style of the older movies.
In Alien, Ripley and all the other people on the Nostromo are basically space truckers, and they act and dress appropriately, which I really kind of like, and I think some of that is applicable to Hawken, too. According to the lore, the people piloting the mechs aren't military trained veterans or anything, these are just regular people, usually lower classes, stuck in a dire situation and are being exploited by those running the corporations for their own financial gain, consequences be damned. These guys don't have a lot of means, they just see a way to make their situation better.

There's one piece of Hawken concept art in particular that I really think matches what I had been imagining so far as this, and that's the piece of art that has a bunch of guys sitting on a mech drinking beer and playing guitar and stuff. That's really the feel so far as the situation these guys find themselves in, as well as the general aesthetic that I think of when I think of Hawken.

As far as what they are wearing once they are in a mech, it could be as simple as the clothing seen above and below; again, they are not always the most well-supported people...


Anyway, there's my two cents.