I am not a designer or even someone with Photoshop skills but I have worn lots of t-shirts over the years.
I know lots of ideas have been tossed around already but I wanted to offer my .02 on a few areas.
White t-shirts are fine for most people and they usually can be purchased in bulk cheaper than a colored shirt. I think they look less expensive though and the larger sizes tend to made of thinner fabric to even the cost. (I know the idea sounds good in sweaty conditions to have a wet white t-shirt on a young lady but look at the reality - Most likely end up with a wet t-shirt on a middle aged sweaty large guy.)
Trust me no one would want to see me in one of those shirts either. I guess you could limit the size of the shirts? But that may hurt sales?
Black t-shirts can be hot in the summertime but Harley riders not only go for the look but it is practical in that it doesn't make the wearer a wet t-shirt contestant and it doesn't show dirt and stains as easily as a white shirt. It can also let the design printed on it to stand out.
As far as Avalanches used on the design I would like to see one of each type used as long as it didn't create an issue of Avalanches crashing into each other or it looking like a parking lot. I think sales of the shirt would increase if each general style was represented such as Cladded, WBH and 07 and up new design. In my opinion if the AVs are too highly modified it may decrease sales since some members feel their truck is not represented.
I thought the idea of the Chevy engine satellite was nice in one of the examples. Maybe it could be used and the website logo could be placed on the globe in it's "home city"?
Attached is a photo of a t-shirt quilt I received for Christmas and it will demonstrate in one photo various backrounds and design colors so people looking at this thread can look for their favorite color combos and it might help them decide for themselves what works and what doesn't?
Good luck with this effort, I'm sure it will turn out great.