The next Halo needs time to marinate; Halo 5 was launched too soon and then cobbled together, TMCC was cobbled together and then launched too soon... the next game has to hit all cylinders, launch with all MP components in place, have an excellent single player experience without copy/paste boss guy all over the place, no weird in between level stand around and chat mission (which were likely fragments left over from scope being cutback).
A second game in development sounds probable at this point, there's no way Phil Spencer hasn't been listening to feedback and there is no way a team that big doesn't have loftier ideas than 'more halo' they want to try.