No you can't create your own missions. The closest thing to that is Forge.
Missions.
No, Halo Reach shows what happened before Halo 1. In fact, in one of the missions in halo reach, you can see Master chief
In my opinion, they're a lot longer than Halo 3's.
if you mean co-op then yes you can
YES! Halo reach has a "Forge" feature, which allows players to create maps, the reccomended map is forge world.
No Halo 4 will not be coming out with a new story line but their is Halo ODST and Halo Reach and If you Buy Halo ODST you will be able to try halo Reach which comes out in 2010.
Halo: Reach comes with a built-in map editor called Forge Mode. You can access this from the main menu of Reach.
practice practice practice :)
if you mean play as aliens you can in halo2,3 and halo reach
no but there might be mods where you might be able too maybe halo reach will let you
The only playable alien race in the Halo franchise are the Elites. You can not play as the Brutes in Reach or any of the other games, with the exception of Halo Wars allowing you to create Brute units.
Yes. Halo Reach will be region free according to 'Domenic' of the Reach team. You will be able to play it on any retail Xbox 360.