Was Tarantino gonna do young Bond? I think that was more of an EON thing, because in the book CASINO ROYALE Bond is already a veteran 00-agent. The whole aspect of Bond being a newly promoted was something one of the producers has been thinking of doing for a long time and tried to go for that story after Roger Moore left the role but Cubby Broccoli rejected it.
Aside from that, I think Brosnan could have pulled it off with the right script and direction. I think by the 2000s he really grew up as an actor and CASINO ROYALE, if done in the style of the book, would have made a pretty good swan song for him. Some of my favorite moments of him as Bond is in DIE ANOTHER DAY, which is unfortunate because he got so much better in the role yet his films kept going downhill.