Everybody Loves Raymond/WMG

Marie vs. Debra

 * One theory that's been posited online is the idea that the real reason why Marie and Debra don't get along is because they both want complete control over Ray. This makes their goals mutually exclusive. Thus, Marie sees Debra as a threat to the control she's had over Ray since he was a child (which is why she's always quick to insult Debra), and Debra sees Marie as an obstacle in her own goal of gaining complete control over Raymond now that they're married.
 * To take that theory a step further: Ray, Frank, Robert, the kids, and even Amy, are all just pawns in a never-ending chess game for power between Marie and Debra.

Debra was abused as a child.
By a man. Because of that,s he subconsiously took her rage out on Ray, because he is the closest man in her life, much like the man who abused her was.

Ray Barone cheated on Debra some time in the past and she uses this fact, along with his desire to keep his family together to torment and generally be as unpleasant as possible in every episode.
Think about:Debra goes out of her way to browbeat and humiliate Ray almost every chance she gets. She won't have sex w/ him on numerous occasions and demeans even loving gestures from him towards her. Unless she is just an anti-social personality and in need of serious counseling (which she may well be) the only RATIONAL explanation is that Ray cheated on her in the past and she uses that fact to "punish" him at every opportunity.

At some point in the future, Ray and Debra will be the "New Frank and Marie."
Given the way the characters interact, it's possible. Further supported by the fact that Marie has repeatedly hinted at the fact that her own relationship with Frank's mother was VERY similar to the present-day relationship between Debra and Marie. Then, there's the fact that Ray shares his father's talent for being a Deadpan Snarker, while Debra shares many of Marie's personality traits (desire to control Ray, among other things). Given these facts, it's really not that hard to imagine that a half-century later, Ray and Debra will seem virtually identical to present-day Frank and Marie.
 * In the episode where Ray and Debra learn that Frank and Marie separated, Younger!Frank and Younger!Marie were played by, who else, Ray Romano and Patricia Heaton.

will get married at some point after the finale
They got together during one of the last few episodes of the show, so presumably they were still together when the show ended. Plus  clearly mentioned during the episode that he certainly wants to get married at some point. And  was clearly enamored of his personality. So presumably they will get married at some point in the future, which means that  and her daughter Molly, Ally's best friend, will officially be part of the family.

Debra's Jerk Sue-ness and the problems later in the series are a Stealth Parody
The fact that the main character is so downbeaten compared to a wife who is obviously in the wrong a lot of the time, but is still treated right, it's entirely possible that we're actually meant to root for Ray and realize that Debra is being extremely unreasonable. The problem? A lot of women in the audience cheered whenever Debra did anything, and sort of ruined the gag.
 * Given that Ray Romano was the driving creative force behind the show and is by no means an idiot this may very well be true. The fans treatment of the Ray/Debra relationship (as well as Peter Boyle's health) however may be one of the reasons he decided not to continue the show despite the fact that it was still very successful. Nobody got the joke anymore.
 * The show did this from the beggining with Marie, who would always win even when it was obvious to everyone sometimes including herself that she was wrong, they eventually gave the same treatment to Debra and at times even Amy.