I really liked Halo 4, and I feel like people really shat over it for nothing. It wasn't a Metroid: Other M for fuck's sake, and Guardians felt like an odd midquel that doesn't really belong anywhere given they didn't do anything with Locke (or at least nothing in the games anyway, and that's all that matters).
This gameplay trailer looks old graphically, but I get that the message was, "This is the Halo you know and love with some minor tweaks. Get in there and start playing". To that I can agree, and with something as rare as 4-player couch co-op nowadays this could really kick off if its gameplay is addictive enough. Even if the graphics are second-rate it would be fine if the game is fun and people enjoy playing it.
As for story, I dunno. I feel like there should be more continuity with Guardians, which in itself had a distinct lack of continuity with 4. The ending part of the trilogy should be a conflict that has been created amidst the first two entries, not ushered in from a side-project strategy game.