If you are really going to do something new, I recommend that you proceed through a work that is very good at this. For example, HALIC(High Availability Lossless Image Compression). It is both extremely fast and has a very good compression ratio, and memory usage is very very low. There is also very strong Multithread support already. I think something like this would be great for the new PNG. Of course, we don't know what the author of HALIC thinks about this.