If they wanted to remake the original Ratchet & Clank on PS4 it would not be at all like the comparisons you guys are posting. PS2 to PS4 would see so much graphical difference, and the gameplay would have to be altered so much, that it would be like a new game.
The voice actor would most likely be JAT, the controls would need to be smoother, strafing would need to come into it, weapon upgrades would need to be introduced (therefore the enemies would need more varieties, and would need to be a lot more powerful).
I'd bet that every level would be redesigned to some extent, some removed, and some added. As I mentioned in another thread, They would have the power to have more space stations, and this would likely include some of Drek's destructive equipment. The original R&C had the levels put together in a way that they had almost no real relevance to the progression of a story. After all, the levels were made, and then a they pieced a story around it. Things are different now.