I can see GM buying chunks; say, Jeep, it's not like Jeep hasn't been passed down and sold around before! Also that's the way GM was formed.
I can also see technical cooperation, like the AMC Pacer/GM Wankel engine (look how great that turned out!) Or Honda 3.5L's in the Saturn Vue, etc.
Another avenue is badge engineering, like GM has done with, oh, just about everyone, Isuzu, Suzuki, Honda, Daewoo, Daihatsu, Toyota, etc., etc.
But a complete merger or buyout? Not worth the anti-trust battle just to win market share. Sooner or later FIAT will dump Chrysler on the next hopeful investor(s). Making it appear that GM is interested might raise interest from other potential suitors.