Because when you talk about balance, it sounds like you got a plan instead of just making a bunch of cool toys and stuff for the players.
Let me just say that i dont give a fuck about the balance.
Sometimes i just want to steamroll something, and what would be better then Space marines with bigass weapons?
Im just dreaming here though.

If you have a certain idea that you are following, might i still suggest going along the lines of Inquisitor partys ?
Like the Inq, battle sisters, psykers, acolytes(soldier archtype) and tech support.
This would still leave room for a lot of interessting combinations and traits without going OP and would work with your inclusion of corruption.
Though i like OP, Grey knight terminators maybe?

On the other hand, a squad of imperial guard mercs with flashy equipment and traits could also rock.
Either in the way of Steiners convict unit or as a commando like the Kasrkin or other Guard veteran units.
But its probablly tricky to choose things from the lore which can actually be represented in the game and are usefull.

• Tallarn Raider: +AP in desert
• Valhallan: +something while in snow (but we don't have snow, doh!)
• Catachan: +level in jungle sectors
• Cadian Kasrkin: +Melee Damage (?)
• Vostroya: +suppression resistance
• and so on...