The Defenders have always been an odd team to peg down. You have a founding roster of heavy hitters, true, but over time that roster has shifted, especially since for long peroids of time, all of the founders had ongoing titles of their own. That naturally created a roster of longtime second and even third stringers who lacked the solo titles that DR. STRANGE, HULK, SILVER SURFER and NAMOR all had (yes, once upon a time, all of 'em had ongoings, well into the mid-late 90's even for some of 'em; only Hulk still has one today).
The basic idea of the Defenders was usually that Dr. Strange would randomly assemble these characters to deal with specific threats, or circumstances would cause that to happen. And hey, circumstance plays a big part in superhero team rosters. New Avengers reminded us of that (Cap was inspired to reform the team based on which heroes randomly showed up for the Rykers brawl).
Naturally, right now most of those core founders are busy. The Hulk just spent months in space, tried to conquor the earth to avenge Sakaar, was defeated by Sentry & Tony Stark, and is comatose in some military base, awaiting his movie to emerge again under his usual status quo. Silver Surfer is back as the herald of Galactus, a backward progression that was one of the only fumbles of ANNIHILATION. Namor is currently tending to plots to manipulate and undermine his regime, during and after Civil War. Technically Dr. Strange is busy on the New Avengers as well, but he has always multi-tasked. I mean he was defender of the cosmos in his own title and always had time for Defenders or other guest appearences over the years.
Of course, these Defenders may be a team of registered Initiative heroes, of which Dr. Strange is not apart of. Then again, he was technically given amnesty, and I could easily see him either relying on technicalities or other people looking away to aid this roster if he had to.
Nighthawk, Hellcat, Valkyrie, Gargoyle, and some of the others have relied on guest spots over the years, and a few of them have popped up recently, such as in THE THING, AVENGERS: THE INITIATIVE, and even MARVEL COMICS PRESENTS. I don't mind seeing them regroup. Especially Valkyrie, who for many years was among one of Marvel's coolest heroines. However, there is a problem; Valkyrie usually rode the genetically bred winged steed Aragorn, given to her by Dane Whitman (Black Knight). That steed was recently blown up by Kraven in PUNISHER WAR JOURNAL.
The question is, who else besides those four? Angel, Iceman, and Beast were actually Defenders for a long while, with Angel funding the team. He's part of the Renegades right now. Beast is busy with MESSIAH COMPLEX (as is Angel). I'm unsure about Iceman; I actually wouldn't mind seeing him do some superheroing elsewhere if given the chance. He's a longtime X-Man who is rarely used to his full potential.
Damien Hellstorm (the Son of Satan, who has been connected to Hellcat for ages) is possible, but would probably need some retools. Moondragon is busy with Annihilation Conquest. Devil-Slayer is dead. Moon Knight is busy being crazy. Andromeda is probably busy helping Namor. Black Knight was also an associate, and he's been off the stage for a while...I wouldn't mind seeing him again. Clea is also probably available and since Dr. Strange is seeing Night-Nurse, may have some sour grapes.