I don't think you'll ever see anything beyond an R72_75 chassis---too wide to ship. Agco seems to be betting on the Massey_Challenger frame for class 8 or 9.I don't agree with it, but that's what I hear thru the grapevine.
A 75 is a class 7. The 05 75's have some improvements such as more capacity in the return and clean grain. It also looks promising in 06 for a class 8. Gleaner is in no hurry because they can cut along side the other machines now (dont't know about the 5 series cat though)
I was under the assumption from some articles that i have read that they class combines by the horsepower, actually ISO kilowatts which is then converted to Hp, of the engine. If this is the case Gleaner could put a larger horsepower engine in the R-75, rename it a R-85 and have a class 8 combine. The article said the proposed class 8 combine would be 375Hp and over.