Antagonist, yes my hasty mistake.
However, simply clinging onto your argument by a simple definition is a slippery grapple on the discussion indeed. A definition is not an absolute here, and we are talking about a film here, a very succesful one at that, but simply a film. It does not have the depth of a book, the sophistication and tightly spun layers carefully interwoven to maintain suspense, arouse curiosity and further the narrative.
We rely on a script, and to bring that script to life we need a performer and to capture that performace we need the technological wizardry of ILM, a camera and some over-educated college geek who listens to Radiohead too much (if thats a bad thing) editing the thing.
The script does not allow the character to work with much. Sidious is the ultimate representation of evil. He is one of many catalysts for Anakins fall, yet he fulfill a role in much the same manner as the witches of Macbeth do. They tempt the main character in much the same way Palpatine does. His character is fleshed out certainly, depsite some of the worst on screen dialogue of the noughties, yet he is little more than a tease for our main characters fall into chaos.
Ian McDiarmid certainly has a nack for playing the sly old man, and does well with the material he is given, he steals the scenes from a passive Christiensen, even if Lucas allows him to go way over the top with his croaking voice effects, yet still he is a very flat uninteresting character (do not confuse the character with sith order). Like the script, the performance is that of the main antagonist, yet little more.
As for the cinematic language, little to nothing could possibly place Palpatine as the main character. The main antagonist perhaps, but I think you can realise that the events of the latter prequels follow the formula and tradition of a Shakespearean tragedy; most is left to chance, not the antagonist. More interestingly, Yoda and Sidious are often pitched facing each other, representing their respective factions and binary oppositions; good versus evil, jedi versus sith, dark versus light. However, more precisely they represent the struggle between the galaxy at large.
In the original trilogy, the Emporer is a sideline villain, and only upon heavy viewing and Vader's betrayal/redemption become expected does he step into his own. And while stepping into his own, he does relatively little. He is not the main villain of Return of the Jedi, that is still Vader, taking any EU or PT conceptions aside.
The reaction shots of Sidious in RotJ give us nothing interesting to take on his character, nothing human to relate to, and believe me when I say this, it is very important in a film that we connect with out main character, and I simply cannot stress that enough.
An audience must develop a connection to the protagonist, antagonist or anti hero of the piece, as an everyman figure or someone of morals. This is the key of mainstream cinema, and I'll tell you now, Star Wars is nothing but mainstream, Hollywood cinema.
We can relate to Old Ben or young Ben in TPM, we are told to (this in my opinion was a failing of the PT) connect to Anakin as he is the ultimate protagonist and Luke. There are the foundation of our connection to this film. In TPM, when Qui-Gon is cut down, we root for Obi-Wan, in AotC, we root for Anakin to save Obi-Wan from Dooku and in the OT, we root for Luke to save the galaxy. You don't relate to Sidious unless your some kind of malign sadist.
I argue semantics yes, but certainly neccesarily. Palpatine is part of a large ensemble, a very large ensemble cast. People might include Padme as a main character, she certainly motivates the main protagonist, Obi-Wan could be considered a main character, given his screentime, but his character was poorly dealt with indeed, left hanging in ignorance for most of the film.
We could say Mace was a main character, yet I speculate many would support this purely because the git carries an amythest lightsaber sporting a BMF plate. Dooku, perhaps is a "main character" in the film. Even with his tiny amount of screentime, still many would say he was. We could also say that Yoda is a main character, for indeed, is he not the ultimate representation of good within the film? Much as the same way Palpatine is as the ultimate representation of evil.
I have the cheek to say that Palpatine is an important, alas not a main character, in the Star Wars movie saga. Why? Because the majority of the saga deals with the story of Anakin. His fall and his redemption. Both his fall and subsequent "return" were orchestrated partly by others, but mostly by chance, circumstance and yes, it feels like the reference is neccesary; the force.
Palpatine was invovled yes, but I believe your giving him far too much credit amidst a story full of badly timed events and dramatic circumstance. Even by definition, Palpatine, in a tragedy, is not a main character, but a background party following an agenda set in his own destiny. Outside of a definition, where it really matters, in the films, Palpatine is even less of the main character.
Keep to the EU books.