Yeah! Single Player would be cool, but only after Multiplayer is Finished.
I think there would be some major problems to solve, if the SP should fell like the original GE.
There are some aspects of the Game which would be not easy to convert.
e.g. in GE64, when the enemy get hit by your gun he stoped to shoot while he's going through some hurt-Animation.
after a few seconds he starts to shoot again.
But this was an escential Part of the Gameplay in GE. Because on this way you could handle a huge group of enemies without even getting a single hit (if you're skilled enougth).
I never saw this kind of Gameplay in an PC 3D-Shooter.
There are just two other Game-Series, with this "Feature": Perfect Dark (PD:Zero maybe?, I never played it) and Timesplitters 1-3.
And if you wanna get a as close as posible to the Original, there are many other things to add.
For instance: many different Dying-Animations, major changes in the KI and so on.
But it would be also cool if there were some things, different to the N64.
Like the Severnaya Mission. It would be cool if this Snow-Tank drives around in the area.
See this link for further informations:
http://mitglied.lycos.de/GameSquare1/reportagen/akte-b.htmAnd here a Version of this Tank done by me a few jears ago in a 3D-Program.