I don't know about the unkown, but I do agree on the not seeing his face, That's a must.
As far as first-person, i liked it in the doom movie, and I would like to see portions of it in the Halo movie. I think with the HUD and the reflections from inside the helmet, that would provide a pretty cool experience.
As for acting, I want to see a hot, sassy girl play cortana, that would be awesome.
for major sargent Johnson, ummm.....Samual L. Jackson or That BIG guy that played the kingpin on daredevil
kinda a tossup for capt. Keyes., and lets face it, the scorpion has GOT to be in there ^_^
hmmm....it is kinda nice to see tallent from Lord of the Rings come to the video game - to - movie scene.