It's Always Sunny in Philadelphia/WMG

Charlie is Charlie Gordon.
The following will spoil the ending of Flowers for Algernon.

Think about it. At the end of Flowers for Algernon, Charlie moved out of New York. Since this was an attempt to move somewhere where people didn't know him, he may have changed his last name to Kelly. He then got a job at Paddy's Pub (also cleaning), and he kills rats because they remind him of what he can remember of Algernon. He didn't remember his parents, so he probably met up with his "mother", who thinks Charlie is her real son because she's, well, crazy. Since Charlie's memory was decreasing, everything said about his past in the show was probably just made up by him, and everybody in the show plays along out of pity. The most clear evidence of this theory are Charlie's impossibly bad spelling, not to mention the fact that he's already borderline retarded.

The finale will be exceedingly dark and humorless and will be about the group realizing that Dennis is a serial killer

 * Or, it will be the funniest episode. The gang will still realize Dennis is a serial killer, though.
 * Is there something to back this up or is this just REALLY Wild Mass Guesing?
 * Dennis being an implied serial killer is somewhat of a running gag on the show, and it's been hinted at repeatedly, especially in the season finale of season seven.

Green Man is Charlie's "safe" persona created to cope with being molested

 * Charlie mentions that being "Green Man" got him through vaguely reference some rough times. Like say, being molested by his uncle.

Dennis is a reincarnation of Patrick Bateman.

 * He's a complete sociopath, a controlling neat freak, a witty deadpan snarker, and an implied serial killer. He even has the taste for eighties music.