That's probably for the best. More time for design & polish. Potentially also more time to take in feedback for the previous game.
H5G wasn't the spin-off to introduce Team Osiris?
j/k
mmhm.... I'd be happy with updated Hood & Miranda, and the rest of the character models just need a higher poly budget based off of the original art design. Sound-wise, as much as I don't fully agree with what 343i has done to alien sounds, H3 was probably the weakest.
As for the rest of the engine... Keep the lighting engine, re-bake for higher resolution lightmaps, revamp the shadowmapping, and add some compute shader stuff (e.g. AO, improved water physics).
Wouldn't mind if they skipped multiplayer altogether... (to avoid splitting playerbase? and also development cost...)