When the guard refresh comes, I imagine GW will be adding AAA kits for several different sub factions, including characters and upgrade kits to theme vehicles after each one. I've always like Krieg, but the modern day look of the Cadians always appealed to me as well. The whole jungle/winter/desert themed armies were always pretty cool too, especially when you saw an entire IG army based around one single theme. It really showed the diversity of the range and gave people a ton of options to make their IG army somewhat unique and personalized.
I don't weather my space marines when painting them. I have always imagined them looking show room ready for pretty much every fight. The bright, bold colors should make them look like sports cars when standing next to thier IG allies, who would be the equivalent of rusted out beat up dump trucks. To that end, if I ever start an IG army, it will be weathered, rusted, dented, and bruised. I want them to look like they've seen a hundred battles. Meanwhile, the marines pop in for a surgical strike and as soon as it is over they are extracted and prepared for the next one. In transit, all their gear is brought back to a perfect standard before redeployment.