Marco has no crew, no purpose and will not play a significant role after Wano. So I believe he will die grandly, sacrificing his life for Luffy's.
Maybe Kaido will kill Luffy (ending the collapse of Act 3), and then Marco will use his phoenix resurrection power to save Luffy, and it will cost him his life in the process. I think that Whitebeard's order for Marco not to let Luffy die will have a deeper weight, so it is very likely that Marco will be facing big fish (Kaido and BM) with Luffy.