Easy. They already have Zenimax studios, right? They have Ninja Theory, Double Fine, inXile, Obsidian...they have Playground, Rare, 343i (even with the massive restructuring), Compulsion, Mojang...
Just invest in your current studios. Help them with tech & resources, have strong creative leadership. That's literally all they need to do. That, and put some real marketing effort behind the big games. Scale back on the Game Pass stuff (either stagger new releases for later into it, remove all the cheap pricing options/loopholes, introduce new premium tiers etc.). Work with 3P devs to bring back IP that make sense, like the High on Life dev working with Conker (as an example).
That's what Microsoft have always needed; buying giant 3P publishers won't resolve core issues if those are down to lack of creative leadership and bad upper management who don't know how to build on momentum or real audience wants organically. It just means that many more teams MS have to spread out budgets and salaries to, possibly meaning more job cuts and reduced resources per team as the belt tightens.
They can make Xbox a huge deal without needing another publisher. They just need to actually try competing, and buying publishers & IP in itself is not competition.