Hey OP, great topic. I really enjoy some of these ideas and wanna discuss your list point by point. Personally though, I just don't see why these initiatives would restore Xbox to the age of market relevancy it enjoyed up until this generation:
1) I do agree that 343 should've been tackling new projects within the Halo universe, but this should've been done before they were formed solely to make a new Halo trilogy. We don't need to go over the decline of the series since then, but suffice it to say, I just think this talented team of devs need to do something that is wholly their own. Having them do Perfect Dark is just another extension of the IP shackles that they are currently under; they'd just be trading one franchise for another one that has been far less successful. And while Halo's lore can be rich to a degree, it just doesn't have the sort of cast and nuance that makes me want to 'explore' it to any degree of depth beyond what we get from the mainline FPS games.
343i should be allowed to tackle a new IP, full-stop. Let them make what they think would be a fantastic game, with no preconceived notions as to what that is or from some entrenched fanbase.
2) Rare pretty much has been a new IP factory for a long, long time. Sure, we got the Kinect Sports trilogy from them, but prior to that they were pursuing a lot of different takes - Kameo, Perfect Dark, Banjo & Kazooie NaB, Viva Pinata - the only problem is that, and this seems to be a reoccurring theme with MS, is that none of these ever 'took off' the way Forza, Gears, or Halo did. Sea of Thieves already has a lot of warning signs that its going to underperform, but more than that, the whole thiing is seemingly built as a service game. I don't think taking the original devs off of a service game they launched would be a good idea if you're hoping you can foster & grow a community after a smaller launch.
What I want to see from MS in regards to Rare is to see them try & communicate to the audience that likes the kind of games Sea of Thieves is emulating, and convince them to try it. Cause right now, it just seems like MS is hoping the Xbox audience will rally around it, and those two groups don't really share a ton of overlap.
3) I don't think the lack of Single-player DLC is what is holding these two games back from thriving. These games needed bigger sales upon their respective release. I think it would be nice to see some sort of follow-through on these projects in regards to their SP, particularly QB, but its unfortunately clear that MS is no longer going to be working with Remedy (since they've very publicly moved on from anything with MS, as cited in the Polygon story) and its hard not to assume lower sales isn't a big factor as to why this happened.
4) This is one of MS' biggest problems, but not necessarily because all these games skipped the Xbox, but moreso WHY they skipped them. For a lot of these developers, they all have relationships with MS on some level, so its not like they just hate Microsoft. Moreover, the reality is that many of these developers know that these games do not sell well on their platform, and they learned this lesson from having released content throughout the years & seeing the level of support & returns that they got from it.
To these publishers, MS just doesn't seem to have done any effort to try & attract more of the audience that enjoys the type of games they produce, ESPECIALLY in this generation. The state of Xbox JP is a travesty, and it seems any of the 'effort' they wanted to put into that region faded once they saw some of their initial sales. The thing is - I get it; the console didn't sell well there, so why continue trying to cultivate an audience in that territory. Honestly, MS probably just never saw Japan having this sort of rise that they wound up having, and had they known this might've happened, they'd have done something. But for many people in the know, the signs of a resurgence were there - several JP devs were adopting western development toolsets & methodologies, and the growing reception of non-traditional games in the West (the more niche titles that we used to see) growing, particularly after Dark Souls.
The situation regarding From Software, the Souls series, and MS/Sony is a perfect example of this. Demon's Souls comes out, Sony misreads its initial launch in Japan & lets Atlus handle the localization to the west. The game does well, but its not this mega-hit, but it is a huge hit culturally. Namco then uses Xbox to better sell the new Dark Souls IP in the west; in fact, Dark Souls 2 & 3 both were featured at MS E3s. Now, seeing all this, you would think MS understands that there is a growing demand for this type of game and see the effect its having on gameplay design across the industry. So, why didn't MS pursue something with From Software? Sony fixed the situation - they admitted they were wrong about Demon's Souls, patched things up with From, and they got Bloodborne out of it. MS was in prime position to secure their own Souls-like from From Software, and didn't do it. This is the sort of niche-genre audience cultivation MS just leaves by the wayside and suffers from it in the long-term.
5) Wholly agreed here.