they should make ODST 2, but loads longer.
Halo 2 should be remade, it was the best halo for online by far.
Halo 3 was pretty meh, IMO it shouldn't be remade as it will die out quickly.
Halo wouldn't be a good MMO, so that idea is completly out of the window, Halo should keep to its roots to keep fans interested.
and meh, the last one can be done easily
Just my 2 cents.