Trumbo is a thumper, but that's about all he's good for. He's got a career OBP of right about .300 and struck out 150+ times last season.
With all his power, he could be a great project for McGwire, but you can't plug him at 3rd. Over 8 games at 3rd last year, he committed 4 errors in only 63 innings. When they stopped using him there, he had a fielding percentage of .714 - eeeeewwwww, right?
As for Seager, yes that would be great, but I think Gibby23 is right. It doesn't make sense for either team. We'd have to propose something with starting pitchers included. Besides, Corey projects better as a 3rd base prospect, and as a better prospect than his brother. That could lead to some ethical dilemma in the future.
Best bet would be a short term vet. Youk would've worked. If similar players are available via trade that's what we should pursue. If not, ask Hanley to move there and pursue a shortstop. If all else fails, give Cruz another shot. If nothing else, he's at least earned an opportunity to show he wasn't a fluke