True Capitalist/WMG

Ghost is immortal.
And his rage is the power source. Many times, he rages so hard that he apparently pukes or gets a heart attack. Yet he still keeps coming back for more. The reason? He's immortal, but he needs to rage HARD everyday to keep up the spell. The Trolls aren't killing him, they're only making him stronger.
 * Then why does he Rage Quit? And skip days sometimes, including, at the time of this writing, a week so far?
 * For his emotional benefit. Ghost usually conserves power by going to Sixth Street, which is why he uses it as an excuse to leave early.

The Engineer is not a Spy, but a Pyro!
Or he could very well be a spy, but... he is unintelligible in the radio broadcasts, even when hosting the show at one point.
 * The Pyro learned how to be a Spy?
 * Actually, it's meant to be that it could be a Spy disguised as a Pyro acting like the Engineer.

Ghost is not only a troll, but also the one behind Bad Lip Reading
It was already theorized that he's a Man of a Thousand Voices- just like the guy behind Bad Lip Reading. Both of them also live in Texas. If he's a troll, it would make sense.

Ghost is an actual ghost
A poltergeist, to be exact. And he keeps the cans around because he needs them to manifest some sort of physical form
 * ... This is brilliant. So all this time, we've been listening to some Texan ghost rant and rave about capitalism as he knew it before he died? How did he get his radio station then?
 * Driving out the previous owner, of course. Imagine being haunted by THIS guy.

Ghost is Cave Johnson
Both characters are prone to rants and have a past with someone named Caroline.

Ghost is a closet Brony
Firstly, despite claiming to hate the show and its fandom, Ghost can recognize quotes, characters, and memes from it, and he's even aware of "clopping" too. Then, we got him getting into a hissyfit over the "Ban all bronies" callers, though this may had been more because of the fact that they somehow got into multiple calls in a row and started to get annoying ("JESUS CHRIS-I MEAN, HOW MANY NUMBERS DO THESE IDIOTS HAVE?!"). Some listeners jumped to conclusions and interpreted it as a defense against them.
 * He says it went downhill in season two. Maybe he loves MLP, but the Bronies aren't real fans.

Ghost is Andrew Ryan
Übercapitalist with a god complex. After being ressurected in a vita chamber, he broadcasts from Rapture, throwing in loads of references to Sixth street and other stereotypically Texan things to throw them off the scent. Alternatively...

Ghost is Frank Fontaine
Goddamn Sploicers!!

Ghost is the descendant of Asura and Kratos
All three of them are known for getting really, really mad. Except Ghost, instead of punching out gods, punches out cans and rants about capitalism.

Ghost is always in such a bad mood because of lack of sleep.
He sure does like to talk about how he only gets a few hours of sleep each night.

Ghost is a pig-literally-, and the Engineer is his caretaker.
He was created as a freak experiment in a laboratory, being giving the human brain of an angry capitalist, and the pig body was harassed by a pony back on the farm, hence his hate of My Little Pony. It also proves correct many conjectures on his nature-he is a capitalist pig, a racist pig, a hambone, and he is not actually Jewish-but he doesn't eat other pigs, so he adheres to Jewish dietary regulations.

Ghost is made out of beer cans.
cans.wav is just him flailing around in anger, and sometimes actually hitting crap on his desk.

Tub Guy isn't actually calling Ghost from the tub, but from the shower
C'mon, we all know it's true.

KaraszKun was hired by Ghost to troll his trolls.
Seeing as how Ghost's fans and trolls really hate KaraszKun, this theory wouldn't be too farfetched.