The lights are on
Firstly: Chief is the only Spartan in his Timeline (excluding those on Onyx). So you would have to introduce Him (Her for all the Girl Gamers out there who love Halo) at a differing time or Somehow explain who another Spartan was made or found which would be a whole lot more trouble than going with Chief.
Secondly: Something to think about is how the partially rampant Cortana will have an impact. I would think that 343 would Change our Favorite Partially-Nude A.I.'s personality to match her new title of Rampancy, which was evident in the Halo Legends shorts Origins I & II (for those who don't have them check them out at goodanime.net search Halo Legends)
Thirdlly: It's evident that they had added a new weapon. whether they will be adding more to Chief's arsenal, or keeping it much the same with fast-forwarded weapons from Reach remains to be seen. Would like to see the weapons from Reach make a return but upgraded to match the time.