First of all, it would be epic if Halo 6 recreated the feeling of Halo 3.
Now I’m going to concentrate on the ending only.
Right away I’ll say it: Chief should die.
He should die heroically.
Imagine this: to defeat Cortana they would have to confine her into some form of "container"or room and lock her out of the Domain, that way she wouldn’t be able to translocate to some other Forerunner facility.
The most epic way for it to end would be with Chief holding Cortana (since she now has a physical Hard-light body) while the others escape the nuke which is being set of.
Just an idea.