You mention that you think Halo 6 would "amass a lot of Xbox360 gamers" but how many do you expect that would be at this point? There has already been Halo: The Master Chief Collection (Which is all 4 original Halo games and even ODST) and Halo 5: Guardians which have both been out for a while now. Don't get me wrong Halo 6 would of course push a lot of sales, but I don't think it would bring in as many older Xbox 360 fans back as you seem to think it would, since I feel like a lot of halo fans that would buy an Xbox for a new Halo game would already have one at this point.
In general the idea of "bringing back past Xbox 360 gamers to the Xbox One" seems to have already been a focus of Microsoft because they've brought back a lot of games that were some of the biggest exclusives on the 360 such as Halo, Gears of War, Forza, Fable (before it got cancelled), etc. They've tried that already but I think eventually it hits a point where even though those games might still be great and still great IP's in general, outside of people that aren't already in the Xbox environment and are expecting those games, other people possibly aren't as interested in buying a new console for them because of how they've known of those games for a while and they aren't really "new" to them anymore because they're the same IP's they've known for a while, even if the newest installments end up great.
I think Microsoft should try to strike a better balance between investing in IP's that are already well known and will please people who already own an Xbox and expected those games such as Gears of War, and creating new IP's that will attract in other audiences that may not have gotten an Xbox otherwise.
They've done a good attempt at this so far this gen such as: Sunset Overdrive, Quantum Break, ReCore, Project Spark, Killer Instinct, Phantom Dust Remastered, and even some upcoming ones like Sea of Thieves, State of Decay 2, Crackdown 3, etc. All either being new IP's or franchises that had not seen a new installment in a long time. (And State of Decay isn't even an old IP, the first game was barely out in 2013).
So while I think they should continue to create and invest in IP's that people love still and still sell well like Halo and such, they should also balance it out with something people haven't seen before. Because someone who might not buy an Xbox for Halo since they've already heard of Halo for such a long time and might just see it as that "same old IP that is always on Xbox", might buy it for one of the newer games already mentioned because it's new and hasn't been seen before to them.
tl;dr: A lot more new fresh games, whether it be new IP's or games in franchises not seen for a while like they did with Killer Instinct, on top of the already popular IP's they have.











