Would it be possable to have a new tier of the backpacks like star tier that requires 4 blocks of nether stars to craft in up grading the diamond one and has double the space of the diamond or just holds more

Yeah, but it will be in 1.12+. I'm explicitly building the API so that it should be simple to add new tiers of backpacks, be it with my mod (internally, or possibly with even some config/json user directed tomfoolery) or, more likely, an addon mod.