I dont know man. Deep Space Nine was doing shape shifting every other week for awhile there, it's just a matter of making it look better than that did, and it's been nearly a decade since that show went off the air.
I don't think it would be that difficult to make Carnage and Venom. Most of the time they would just be normal looking characters anyway. Ok, Carnage not so much. But if they can get Doc Ocks tentacles to look normal, they can get Carnage and Venom.