I wouldn't mind seeing Techland and Undead Labs linking up to work on a lil somethin somethin

Ya, maybe a State of Decay shared world style game. Undead Labs has some good game system ideas, but I think Techland could help them a lot with actual moment to moment gameplay. You could do sort of zombie version of Destiny, with deeper RPG, and survival mechanics.

Booty confirmed in an interview a few months back that the next State of Decay would be a AAA shared world game.

Was not so good actually but I was hyped like hell about it :) But the potential was there, I really like the idea and the squad based game where you can switch between characters when required.

A Halo RPG would be amazing and I agree that I would love to see Obsidian doing that; the universe is so rich.
On the other hand, I already expect (and hope :)) Halo Infinite to be fully open world with some RPG elements already :)

I don't see how is this collaboration differ from one of them getting a support studio and extra resources to help shape up their next game. This isn't pure math where 1+1=2. 

In collaboration between Coalition and Initiative I can see TC helping with the shooting mechanics and cover mechanics if needed. They might add graphical experience as well. What would Obsidian add to InXile or vice versa? Minor improvements? I can see a value in one of them advising the other after testing the game but not in collaboration. I can only see a collaboration between these studios if Xbox lacks budget and they want to create something bit. Luckily,  this isn't the case. I'd rather each one of them sign with a 300 man studio to help in coding. We'll get two excellent games instead of one. 

I can see Undead Lab going L4D stule for future game, but instead of shooting your way to the finish line you go RPG way. You start with small introduction about how the city turned into Zombie land and then you start building you're base through quests by talking to survivors locating weapons, building materials, vehicles, chemicals and other things. They can collaborate with InXile to write the main stories and quest trees. They can copy Witcher style side missions where surviving mothers ask you to look for their children.

I said L4D style because I can't think of a way to have a 30+ zombie game story driven without being repetitive. They can create 4 smaller cities that needs 6-10 hours each to go through the main story.

If we look at it from business side of things, I hope Microsoft won’t sell the IP to Platinum. Because judging by W101 announcement, Scalebound will come to PC/Switch/PS4. MS will make fools of themselves if they sell an IP to Platinum.

Wonderful 101 feels like a smaller passion project.
I'd fully expect a multiplat Scalebound to release on both XB1, PS4, Switch, and PC considering its scale (no pun.. ok a little).

Edit: Just saw Ryuu's update. Nevermind.

Xbox Era podcast leaked the W101 kickstarter since january 12. Almost a month before anyone else. They also leaked the HZD for PC.
We should probably pay more attention to their rumors from now on.