If Marbury was added to the roster, the only big issue would be a 6'10 (or taller) center on the bench. And as you know, most tall, talented guys are either on expensive, bad contracts, or just not good enough to play on this team. And I'm not even sure that the guy I'd like to see, would do much better than Glen Davis, but he's got more size. Plus to me it seems like a realistic trade.
http://games.espn.go.com/nba/features/traderesult?players=138~3200~234&teams=15~15~2&te=&cash=Francisco Elson already won a ring with the Spurs in the 06-07 season, so he's been in the playoffs before as a backup (He actually started some of the games too). He's never been a beast on the boards or a scorer for that matter, but I think he could be solid with Powe as a part of the bench unit and most likely fill the role of PJ Brown. I don't know much about his game though, so I don't know if he's got an outside shot - But hey, it's better than seeing Davis taking jumpers with a droprate of 30 % anyway.
And why would Milwaukee trade this guy? Well, he's only playing 10 minutes a game right now, and he's no cornerstone of their team. He might be somewhat of a mentor for Andrew Bogut, but I think the Bucks would let him go without much hesitation. They've also got Malik Allen and Dan Gadzuric for this role, so god knows they've got more than enough centers. They'd get Big Baby Davis as a backup PF with some upside that can produce more than Elson does as this moment, plus he's a young guy. Milwaukee aren't going to compete for playoff spots this season and I doubt they see Elson as a part of the franchise future, given that the guy is 32 years old..
Edit: The trade would also work with JR Giddens or Gabe Pruitt instead of Sam Cassell.
I also just read that Elson and Garnett have a history from the playoffs, so perhaps this wouldn't he a good solution anyway..