His visit to the Bay Area followed a quick trip up to Seattle. As is often the the issue, Abraham will likely come down to price. He was a salary cap casualty in Atlanta and I can't imagine he'll be looking to take a sizable pay cut in 2013.
Of course, given that the NFL is trending younger thanks in part to the rookie salary cap, he might have no choice. This would benefit the 49ers who could really use some solid depth behind Aldon Smith and Ahmad Brooks. For the 49ers, Abraham would be best suited as a rotational outside linebacker in the base 3-4, and then at defensive end when the team switches to the nickel.
Someone made an interesting point on Twitter about how the team could potentially add some 4-3 looks to the defense. The team has not run a hybrid 4-3/3-4 since the Nolan days, but if Fangio is looking to evolve the defense a bit further, that is one option. It's my understanding that at the Sloan MIT conference, Paraag Marathe mentioned that one reason the 49ers were inclined to pay big money for their two inside linebackers is that they felt if the team ever did switch back to a 4-3, Patrick Willis and NaVorro Bowman could work plenty fine in that.
Conceivably they could go with Bowman, Willis and Brooks at the linebacker spots, and then Aldon Smith, Justin Smith, Ray McDonald and John Abraham on the line. It could be just one more look from an already potent defense.