Graphene thermal paste could only be for very high end use, at it's price prevents it from being a low-medium end possibility. I doubt anybody who is up in the high end spectrum would be using an aluminum heatsink. CL Liquid Pro is also corrosive between copper/aluminum yet it's done quite well.
Yeah, keeping a flat IHS and HS is always an important factor, but Thermal Interface Material has always been and probably always will be the weakest link in cooling systems so there will always be people trying to make better TIMs.
I just remembered, there were some people working on a "Graphene Foam" material. It was essentially graphene bonding in a random 3D "Foam" like pattern, as opposed to it's normal flat pattern. They stopped research/development on it after they made some, because it was incredibly brittle. At least, last time I looked into that was the case.